Ending makes no sense--spoiler alert
Why does Michael have Carlo killed, considering the whole first part of the movie is based on Connie's and Carlo's huge wedding? It makes no sense he would kill his sister's husband, even though he was a rat. The lie he told Carlo would make much more sense: that he's out of the family and going to Vegas as punishment. So Carlo wasn't that dumb to confess, believing that Michael would never kill his brother-in-law. But wasn't that smart not to hedge his bets.
But G2 was totally ridiculous, with the whole Fredo thing, Senator thing, etc. Mediocre followup script, though a much bigger budget riding the coattails of G1.
https://www.youtube.com/watch?v=8vZx7yF_a7M