I don't see how it glorified the mafia. I mean, look at Vito. He got people wanting to kill him, his eldest son is violently killed, his youngest son is forced to escape the country for his life.

Then look at Michael: his first wife is killed, he ruins the relationship with his second wife, one brother is violently killed, the other he has killed himself, his daughter gets killed and in the end, he dies completely alone.

So I think they made it clear that it sucks to be a gangster.