Goodfella:
In the book, Sonny goes over the alternatives to kill Sollozzo by someone other than Mike. Sonny himself can't do it because the Family cannot risk losing Sonny while the Don is recovering from his gunshot wounds. Fredo is still in shock from his father's shooting. Sollozzo would never let Clemenza or Tessio get close enough to have a sit down. So it has to be Michael who does it. And don't forget, at least 3 times Puzo described Michael as feeling "that delicious chill" inside when defending the honor of his family.

But if someone else had killed Sollozzo instead, there would be no real reason to even have Michael in the story. I say that because this part of the plot is so vital to Michael's character. Without Michael deciding to kill the men who tried to kill Vito Corleone, his life would have been similar to what he was like at Connie's wedding: warm-hearted, quiet, a war hero, and not wanting to have anything to do with his father's business.