For people who had not read the book, they would probably have not clocked on in the first place that Carlo was involved, as its it not obvious to the first time viewer, so when he is whacked at the end after being made the right-hand man, its comes a fantastic surprise.

But when you watch it again knowing that Carlo is the betrayer its even better cos you can watch Michaels calculation by giving Carlo a false sense of security.

Couple of things, How did Michael piece together the events of Sonnys death when he was in sicily at the time, connies whimpering??, Second did the Don know of Carlo's guilt? thirdly, if they both did, how much restraint did they need not to throttle him on sight.