There is so much of Al's history in the novel, and it's one of my favorite parts. Although he was a cop, he was a very violent and physical man. He often flouted the law, and sought his own form of justice. That made Vito realize that Al Neri had a different destiny, and when the opportunity to turn him came along, the Corleones jumped on it. As DC pointed out, Al Neri was theirs forever after that.