In The Godfather, the location of meeting between Michael and Sollozo was known to the Corleone family. Wasn't it much easier to let a button man kill Sollozo? They already had Tessio waiting for him at the restaurant to collect him. This wouldn't have required Michael to be exiled to Sicily.
In the Godfather, the decision to have Michael kill Sollozzo and McCluskey was a complicated decision for the Corleone family.
Sollozzo was a powerful figure in the underworld, and his death would have serious consequences. The Corleone family knew that there would be reprisals and consequences if they were seen as the ones responsible for Sollozzo's death.
By sending Michael, an upstanding veteran to meet with Sollozzo, in a known (therefore safer) public place makes this assassination attempt easier to disguise. Sollozzo would not be as suspicious.
IMO, Michael's actions during the assassination helped to solidify his position within the Corleone family. By taking out Sollozzo and McCluskey, This act alone was a turning point for Michael's character and marked the beginning of his transformation into a true mafia boss and plot-wise, it is hard to continue this story having Sollozzo killed by a buttonman; a dramatic turning point for Michael character development would not be feasible or convincing.
A failed assassination would have been disastrous. The Corleone family needed to maximize the probability of success.
The Corleone family needed someone they could absolutely trust to do the act. Who knew who was secretly working for Sollozzo or one of the other families? Who could the Corleones trust to do the deed? Michael was above reproach.
Other members of the Corleone crime family quite possibly could be known to Sollozzo or to McCluskey. They therefore could not be waiting at the restaurant without drawing suspicion, nor could they enter after Sollozzo and McCluskey arrived. Sollozzo and McCluskey did not perceive Michael to be a threat.
It was Sollozzo who asked for Michael be sent to the meeting1. In his point of view, Michael is not dangerous, and even though he's from the Corleone family, he's not in charge of anything, not business-related. Being no threat, he's the one left to discuss. It can't be Sonny or Tom because of their position in the organization. And it can't be a bit player or a buttonman2.
Michael is invited to the meeting, they'll allow him to come close. They think he's on his own, with no backup and no help. This wouldn't happen with anyone else, except for Tom.
They checked (on the bridge to New Jersey IIRC) that they weren't tailed. So, Sollozzo and McCluskey felt safe with Michael, despite him being a war hero, he's no cold-blooded killer. They frisked him, and they thought no one would be able to spot them in that place.
Knowing they don't fear him, but strongly decided to avenge his father shooting, Michael intends to make the move and kill them. Doing this, he not only defends his father and family, but, as sicilian, retaliates against the people who meant to destroy his family. Sollozzo and McCluskey can't expect that because he's not seen as an man of action.
He was the natural choice to do that though3, Sonny and the others realized that, helped him, set up the meeting, the escape to Sicily. That scene is also one of the most pivotal events of the story, where Michael turns to being a real mob, and not just the "son/brother of".
Why did Michael have to kill Sollozo? I've always seen Michael as the heir of Vito, not only because of the father/son relationship, but because they also share the same Sicilian blood and type of reasoning. In The Godfather II, younger Vito Corleone was 'punched' twice by Don Fanucci. First at the grocery store, when he got him fired to force Abbandando to hire his nephew. Second, when he wants Vito, Tessio and Clemenza and their small illegal business to pay a tax. Insulted and threatened, Vito comes up with a sophisticated plan to not only kill Fanucci, but also take the lead over Tessio and Clemenza while keeping their money for the tax. About Michael, the 2 'punches' are the attempt at killing his father, and the one in his face, from McCluskey, with the knuckles, in front of the hospital. He then comes with this also sophisticated plan to not only kill Sollozzo and McCluskey but to strenghten his family position like his father did.
Additional note. 2 There's a comment asking "why a button man couldn't be there already, incognito. There were several other diners in the restaurant; why couldn't one of those be a plant? After all, we know that one of the Corleones' people had been in the restaurant earlier, to plant the gun. Compared to Michael, a 'professional' would be much more experienced, as well as not being under as close surveillance".
If a hitman kills them both, it starts a war the Corleones might not win. As stated during the meeting with the 3 brothers, no one had ever killed a police captain, and doing so would bring the whole police forces against them. It would then be 'all of them' vs the Corleones, and no doubt the other families would take the opportunity to push to make them fall quicker and harder.
Once the journalists start the hullabaloo of articles on a daily basis (even twice a day at this time, morning and evening news), it's easier to explain a plot and how the Corleones were framed by Sollozzo and McCluskey. They threatened Michael and his family, and he had no choice but defend himself in the restaurant. They can turn this assassination into self-defense.
3. This article backs it up: "Soon after, Sollozzo seeks a meeting with Michael to resolve the hostilities, although at Michael's urging, the Corleones view this as an opportunity to kill Sollozzo, which would, of necessity, also involve killing McCluskey. Michael successfully convinces Sonny Corleone, Tom Hagen and other leaders of the Family that the usual strict Mafia prohibition against killing police for fear of bringing down the retribution of the authorities should not apply in this case, since McCluskey is a corrupt cop on the mafia's payroll, and who is involved in drugs. He further says that this would make a sensational news story to be given to newspaper people on the Corleone Family payroll after the fact. Although the double murder, including that of a police captain, brings an official crackdown on organized crime, the subsequent leak of information about McCluskey's criminal links to Sollozzo - as Michael correctly predicted - gets wide coverage in the newspapers and takes some of the attention off Michael and his family." from The Godfather characters
1. at 1:10 in this scene of the original movie.
From the comments of the video, I'll quote some very interesting points.
> 1. "Micheal was a marine so he knows war time strategies. This is such a pivotal scene. He goes from civilian college boy to crossing over that line of no return. Brilliant!" (Natalie)
> 2. "that was the last time anyone laughed in Michael's presence." (Ghali)
> 3. "When the camera centers on Michael and begins zooming in on him, you can see Michael becoming the Michael he never wanted to be. Michael: 'It's not personal, Tom, it's strictly business.' " (Nick Wride)