Michael survives an attempt on his life at the beginning of The Godfather Part II. The gunmen miss him and attempt to flee. We then see that they have been killed.
Who killed the bodyguards?
The first question that needs to be answered is who tried to kill Michael. Then, some theories as to who killed the would-be assassinations can be drawn.
As we know by the end of the film, Hyman Roth is behind the hit on Michael. There are a few reasons for this, the largest of which is resentment at Michael having killed Moe Greene in Godfather I.
To do this, he uses Johnny Ola, his right hand man. He sends Johnny to talk to Fredo, Michael's brother, and has him manipulate him into giving information that can be used to attempt an assassination. We never find out what this information is. Whether Fredo was callous enough to give direct information that could be used, or simply angry and careless enough to let slip something innocuous, we don't know.
This exchange is all we have on it. You can decide for yourself how "in the know" Fredo really was:
Fredo Corleone: I haven't got a lot to say, Mike.
Michael Corleone: We have time.
Fredo Corleone: I was kept pretty much in the dark. I didn't know all that much.
Michael Corleone: What about now? Is there anything you can help me out with? Anything you can tell me now?
Fredo Corleone: They've got Pentangeli. That's all I can tell you. I didn't know it was gonna be a hit, Mike. I swear to God, I didn't know it was gonna be a hit. Johnny Ola bumped into me at Beverly Hills, and he said that he wanted to talk. He said that you and - and Roth were in on a - a big deal together and that there was something in it for me if I could help 'em out. He said that - He said that you were bein' tough on the negotiations, but if they could get a little help and close the deal fast, it'd be good for the family.
Michael Corleone: You believed that story? You believed that?
Fredo Corleone: He said there was somethin' in it for me. On my own...
So now we know the three people implicitly involved in the assassination attempt. As to who killed the failed-assassins, there are a few possibilities:
Fredo - He could have killed the assassins out of panic whenever he realised the hit had failed. This could have happened two ways - he might have given Roth information allowing him to access the compound, or allowed a mole in, and once he realised what transpired, murdered the assassins inside the compound out of fear. Or, of course, he could have been well aware all along a hit was going to take place and killed the assassins out of self preservation. However, if Fredo had killed them, it raises questions as to how he knew exactly where they would be (and how he could kill them with no other guard on the compound knowing). If Fredo did kill the assassins, it's difficult to imagine he wasn't well aware there was an assassination plot.
Fredo's Men - Fredo is a caporegime, so he has a lot of men under him. He could have had one of those men kill the assassins (then either hushed his man with money or killed him to silence him). However, the biggest issue with this idea is the notion that Fredo could offer someone more than Michael could (or could intimidate someone more than Michael could).
Michael's Guards - It is of course possible that Michael's guards killed the assassins. These guys are mafia men - they're not trained soldiers. If they cornered an assassin and the assassin raised his gun, it's unlikely they would be aiming for the shoulder or foot. It's perfectly plausible that a scenario like this occurred and Michael's men killed everyone.
The movie (or books) don't ever actually say exactly who killed the assassins, but this conversation occurs after the hit:
If we catch these guys do you think we'll be able to find out who's backing them?
That's not the catch -- unless I'm very wrong, they're dead already. They're killed by somebody close to us -- inside. Very, very scared they botched it.
What about your people ROCCO and NERI? You don't think that they had something to do with this.
You see -- all our people are business men, their loyalty is based on that. Now, one thing that I learned from Pop was to try to think as people around you think. Now on that basis, anything's possible.
So Michael certainly suspects an inside man. Given that we later find out Fredo betrayed Michael, that most likely scenario does seem to be that Fredo himself killed the assassins.
To reiterate however, all of this is just speculation, as it is never confirmed in either the films or books.
First let's rule out who didn't organize and execute the hit attempt and then kill the assassins.
Fredo. Fredo is too stupid, inept, weak and cowardly to pull something like this off. Additionally, Fredo actually loved Micheal, though he was filled with resentment towards him over his being passed over as the family boss. It is unlikely that any men whom Fredo commanded would have participated in such an scheme. More than anyone else, they knew what a putz Fredo was and such a scheme would never work, would blow up in their faces and they would face Michael's terrifying wrath.
Frank Pentangeli. Pentangeli was a simple man, loyal to Michael and he hated Hyman Roth and Johnny Ola. Why would he help out Roth and Ola. Furthermore, Pentangeli is a family member of long standing. He understands how capable and ruthless Michael can be. He is simple, but not stupid. He knows that he would lose everything in a showdown with Michael.
Al Neri. Too loyal. Michael's underboss and "Luca Brasi". A man of limited means and intelligence, but with great respect from Michael. Ruthlessly efficient and happy with his role as the Corleone family ultimate enforcer.
Tom Hagen. No way. Loyal unto death. His highest complement is to be recognized as a true brother to Michael and Santino. Too smart, loyal and affectionate towards Michael to pull something like this. Furthermore, Hagen abjures violence. His policy is to avoid violence as much as possible. He would make every effort under the sun to resolve differences through negotiation.
This leaves us with the one man who is both ruthless, close, but somewhat obscure: Rocco Lampone. Michael confesses to Tom that Tom is the only many whom he truly trusts. He understands that Rocco's and even Neri's loyalty extends only as far as their "business" interests coincide. We are not privy to any tension between Rocco and Michael but we have to infer that their might be some. Rocco is a street dude. Corporate business is not his ken. He probably itched to be a street boss back in New York again. Rocco was in charge of security in the compound. He could easily have let the assasins in, positioned them so they could get a clear shot at Michael and then in the post assasination rendezvous killed them both. Didn't Rocco, or his men, find the dead assasins? Furthermore, a smart conspirator like Hyman Roth would never rely upon just one operative. He would be working at least 2, maybe more towards the same goal, all the while keeping the co-conspirators in the dark about each other. Fredo had no idea that Rocco was involved the the hit.
In the final scenes Michael sends Rocco on a suicide mission to kill Hyman Roth. It was evident that there was little chance that Rocco would get out of this deed alive. Tom Hagen mentions this fact that Roth is surrounded by police, body guards and tons of media. So why did Rocco undertake the assignment. Most likely Michael confronted him and told him that he knew that Rocco orchestrated the hit, but he offered him a deal that Rocco couldn't refuse. Undertake this suicide mission to kill Roth and if he was successful, in both the assassination and escape, he, Rocco, would get a pass for his betrayal and a little financial settlement, he would be out of the family and the business even forever. If Michael got wind of him in the business again he would have him killed. Rocco would essentially be put in retirement. If he was killed during the attempt, his family would be taken care of. If Rocco refused the assignment Michael would have him killed the minute he left the room. This was an offer Rocco couldn't refuse. Rocco undertook the suicidal assignment and was killed in the execution of it, but he achieved the goal of assassinating Hyman Roth.
So, Rocco Lampone organized and executed the hit on Michael and killed the assassins afterward.
It's 100% Rocco. He asks where Michael is after Michael leaves the compound and Tom has to shout him down.to make him re focus. Rocco is guilty . And faking concern for the now safe Michael.
Here's the brilliant part. Those 2 dead shlubs in the sewer? Rocco had them killed BEFORE the assassination on Michael. Perfect forever silent patsys. Probably two transients whose throats he had slashed.
A couple comments are clues to me. "A couple guys out of NY" "We won't get anything out of them now" The NY comment sounds like disinformation, so I'm guessing the guys are from Los Angeles and are Johnny Ola's guys, thus Hyman Roth. That's how Michael figured out it was Roth. Whoever said that had knowledge. It could have been Rocco, and Roth might not have known Rocco so he wasn't recognized by Roth at the airport. I don't think I'll ever understand the suicied hit at the airport. The above explanation of an ultimatum given to Rocco is as good an explanation as any, Rocco must have had family or other interests that would be preserved if he did the suicide hit.
In the scene with the bodies in the stream I believe you can see a slit throat because there were no other gun shots. I also don't think Michaels Rocco and Neri did it , they seem very loyal. So maybe Johnny Ola (from Sopranos fame) some how wormed a couple boys in there but the powers that be didn't go into it.
IMO, the most plausible answer would be that Michael's men hunted for, then discovered the shooters, and killed them. However, we don't hear any more gunshots after the bedroom scene, and Michael specifically ordered they be kept alive, so the next most plausible scenario is that pentangeli left one or more men hidden inside the compound (after he left for New York) in order to silence the shooters. It's also possible the shooters killed themselves, especially if they realised they hadn't been successful. I don't see Fredo killing anyone, considering how he handled the attack on his father (Godfather I).
This question has always bothered me.
It is correct that Fredo doesn't have the stones to kill, maybe, anybody. A killer, and a successful Mafia soldier, has to have a cold-bloodedness that Fredo simply doesn't possess. That has been seen many times. Fredo could have opened the drapes, though. Pentangeli leaving men behind (even one) -- that's a pretty good theory. It would explain why Pentangeli is defensive later when Michael comes to see him at what was Don Vito's house. "Let's hit 'em now, while we got the muscle" - that line is a little desperate on Frankie's part.
Come on, there's no way Fredo had the stones to kill anybody: remember the way he fumbled his pistol when Sollozzo's men tried to kill don Vito? Also, as mentioned above, there's the sheer physical impossibility of Fredo being able to find them, much less kill them, without being noticed -- they guy's no James Bond or Ethan Hunt, he can't even control his wife. Also, Fredo didn't live at the Tahoe compound, so why would he have brought his own muscle to his brother's house, and why would Michael allow them to mingle with the regular security detail even if Fredo had brought them? Either it's a plot hole, or the simplest explanation is that they killed themselves in order to avoid capture; in fact this sort of self-sacrifice is shown later when Rocco kills Roth, knowing the police will probably kill him.
From Fredos answer to Michaels question about what he could tell him, we learned that Fredo knew they had Pentengeli. Based on that alone it indicates Fredo was in on the whole plan from the start, including opening the drapes in Michael's bedroom, the hit and getting rid of the would be assassins.