Some of us here have puzzled over Michael's requiring Carlo to confess. Some have said that it was almost a "moral" thing: Michael had to be sure--to hear it from Carlo--before he had him whacked. I doubt it. I think it made for a great bit of drama, in the novel and especially the film.

In the novel, Michael says he doesn't want to see Tessio at all. Tom was the bad news deliverer, and once again, we had a nice bit of drama. Frankie had already "confessed" by appearing before the Senate. His suicide was a quid pro quo for the safety and well being of his family. IMO, Michael was not just demanding vengeance--he was protecting himself. Although Frankie's credibility was finished, the FBI wasn't going to give him a free ride forever. They'd have pressed him to give more info that they could have used to build a case against Michael and/or Tom, Rocco and Neri over time.