They conveniently left out Carlo's funeral, which I presume would have been attended by many of his wedding guests. Point is the wedding was a huge event and the center of the whole first part of the film. Then the guy not even part of the family business becomes the Don and kills the man who got married: his brother-in-law?
The other aspect I didn't get was how and why exactly did Carlo set up Santino? He beat Connie knowing she would call Sonny, knowing he would leave the secure compound and drive there using that tollbooth…that's a lot of contingencies.
Just because Sonny beat him up? Wouldn't he know that would spell his own death? They should have gone further into the setup.
I mean I know in real life the Mafia used to hit their own, and not many outsiders. I know that Sammy the Bull killed his brother-in-law. Maybe he was copycatting what he saw in G1!
Mean people suck.

Carlo set up the future Don of the family for execution…he had to go