How we cite our quotes: All quotations are from The Godfather.
Quote #4
DON CORLEONE: I said that I would see you because I had heard that you were a serious man, to be treated with respect. But I must say no to you and let me give you my reasons. It's true I have a lot of friends in politics, but they wouldn't be so friendly if they knew my business was drugs instead of gambling, which they consider a harmless vice. But drugs, that's a dirty business.
Don Corleone doesn't like drugs because they're overtly harmful. (Of course, people get addicted to gambling too, but he sees that as being less negative.) Even though he's a criminal, he surprisingly has things that he is and isn't willing to do. Drugs don't jibe with his sense of tradition or mission. He views himself as being fundamentally just, if technically subverting the law.