We know Fredo betrayed Michael by working with Hyman Roth and Johnny Ola. What exactly did he do though?
He says he didn't know it was going to be a hit and that he was only trying to help the family. So what exactly did he do? Did he let the gunmen into the complex? Did he provide information about how the gunmen could get into the complex? If not then what?
I always had the impression that Fredo just provided Roth with some extra information in order for Roth's supposed deal with Michael to succeed and for himself to make a good figure, not knowing about Roth's plans against Michael. Maybe he even wanted a bit of a little advantage for himself out of the deal, feeling ignored by the family though being the older brother. So I think he was more careless and naive (which Roth used to his advantage) than having actual bad intentions against Michael. At least that's what Fredo says about it to Michael:
Fredo: 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: You believed that story? You believed that?
Fredo: He said there was somethin' in it for me. On my own...
While we don't neccessarily need to believe him (as he could just be pleading for innocence before Michael), this seems pretty much in line with how his character is depicted throughout the series. He always seemed to me as not the brightest of the brothers and a bit naive. It doesn't seem too unlikely that he would be a bit too careless with information when he thought he could make something "on his own", without deliberately endangering the family.
So I for myself believe his pleading. I don't think he ever intended Michael to be hurt or the family as a whole, even if he maybe wanted to make a little advantage behind his back. This just wouldn't fit to his character as depicted, I think. But this is all just my own impression without much substantial proof and without providing much information of what exactly he told Roth then (and I admit I haven't seen it for quite some time).
Take a look at Wikipedia. It is explained as much as it is possible to deduce from the movie:
Fredo later betrays Michael when approached by Johnny Ola (Dominic Chianese), an agent of rival gangster Hyman Roth (Lee Strasberg), during the negotiation of a business deal between Roth's organization and the Corleone family. Ola and Roth claim that Michael is being particularly difficult in the negotiations, and Fredo secretly agrees to aid them in exchange for compensation; the film never reveals what specific assistance Fredo provides Ola and Roth against Michael, or what he receives in return. In any event, Roth's men use this information to make an attempt on Michael's life at Michael's home.
While in Havana negotiating with Roth, Michael realizes that Fredo is the family traitor he had been looking for. Despite twice telling Michael that he had never met Ola, Fredo drunkenly lets slip that they had met in Havana earlier that year. Michael confronts Fredo later, giving him the kiss of death and telling him, "I know it was you, Fredo. You broke my heart. You broke my heart!" In the ensuing fray after dictator Fulgencio Batista's flight from Fidel Castro's rebel army, Michael pleads with Fredo to come with him, but a frightened Fredo runs away. He is eventually tracked down and convinced to return home.
In the original script Hagen says Fredo thought it was going to be just a kidnapping, giving background to why Fredo says "not a hit" in the completed film.
MICHAEL: I want you to reach Fredo. I know he's scared, but have one of our people reach him. Assure him that there will be no reprisals. Tell him that I know Roth misled him.
HAGEN: My information is that Fredo thought it was a kidnapping. Roth assured him nothing would happen to you.
Fredo was the Underboss. Number 2. With Michael out of the way, Fredo is the new Don and Roth gets control over the Corleone empire.
Fredo is naive and stupid. But, he is a mobster and make no mistake about it, he is very greedy and gets pushed around by everyone - Moe Greene, Sunny, Michael, his alcoholic wife, etc.
Fredo was in on the hit. He had to be. How did Fredo know about the plan to have Senate Subcommittee nail Michael for perjury, if those details weren't shared by Roth and Ola? If Roth and Ola were comfortable sharing that information with Fredo, then they were comfortable knowing they had Fredo's trust. Fredo didn't come clean with this information until AFTER he realized Michael knew he was the traitor.
Sorry folks, but Fredo tried to have his own brother killed and got what he had coming to him. Films draw a lot of analogies with Roman Empire and this is how the Romans did it.
Well, it could be said that he warned Johnny Ola that an attempt was going to be made on Hyman Roth in Cuba, which would explain the guards catching the assassin in the act.
Secondly, it could be argued that Fredo left the blinds open in Michael's room, to help out the assassins spot Michael, although naive Fredo didn't think they were going to try and kill him "just scare him". He would have given Johnny (uncle junior) Ola details on the family business as well including Michael's comings and goings. Michael warned Fredo in the first film not to side against the family so his death was inevitable.
Fredo killed the would-be assassins. When Michael talks to Hagen after the shooting, he says "unless I'm terribly wrong, they're already dead" about the gunmen, "killed by someone close to the family". Fredo ends up being the someone "close to the family".
He knew all along what was to happen.
In The Godfather Part 2, there is a scene showing Fredo being sick as a child, possibly alluding to his abnormalities. Being a liability, he was ultimately sent to early Las Vegas and kept in the dark about the family business. He obviously was having nefarious conspiring conversations with Roth and Ola and was out for himself. Michael didn't need to kill everyone, just his enemies, which included Fredo. Tom was the only true loyal one in my opinion. There is supposedly an extra hour of footage out there which might explain more. The comment about opening the blinds at the home before the attempt on Michael was interesting.
