Both.
Many, as Don pointed out came from poorer backgrounds and saw becoming a 'gangster' as the only way to make money. Also, I'm sure anyone can see the advantage or allure of being respected by people as well as feared by them. No one wants to be disrespected or feel like everyone knows they are better or stronger than them. Being a rich, powerful criminal gave them the security they wanted.
Another reason might be that they were born into a family of criminals and see it as a normal life, or one which they have grown up seeing and like the look of.

Pippi