Everyone obliged by being conveniently out in the open, for their easy killings
He had talked to Barzini No doubt he had to go but cat out of the bag with his well before killing

I think only Barzini and Tattaglia were killed, in the novel Michael was the only one left standing in the movie No doubt everyone knew Church attendance airtight alibi or not Dons don't personally murder except Vito's of Don Ciccio but it was strictly personal!