I think some of the most interesting, compelling heroes in romance novels are the ones who began life as a villain in another book.
One of my first heroes was Urien Fitzroy, in THE WARRIOR'S QUEST, who began life as the yes-man of the villain in my first book, A WARRIOR'S HEART. In A WARRIOR'S HEART, Urien's job was a simple one: to give the villain someone to talk to, to explain his dastardly plans.
But then, as I've told many people, he suddenly developed a conscience. It was one of those times I was writing along, just going with the flow, and boom! The man was not happy about what he was supposed to do.
To my mind, a true villain may occasionally feel the prick of conscience. If he's intelligent and not mentally ill, he knows right from wrong. The difference between a villain and a hero is that the villain is able to ignore those little nudges, or rationalize them away (it's a dog-eat-dog world, they'd do the same to me if they had the chance, the world owes me). A hero will do what his conscience tells him is right.
For a villain-to-hero transformation to really work for me, the potential hero has to be bothered by the pricking of his conscience before he even meets the heroine. He tries, but he can't quite rationalize his choices. He does what he does because he thinks he needs to, but his methods do keep him awake in the wee hours of the night.
So it's not "the love of a good woman" alone that changes him. For me to believe he's truly changed for the better and that the change will "stick", I have to believe there was already the potential for change -- a willing heart, if you will. He just needed the incentive and the strength falling in love with the heroine provides.
Because the change from bad to good hasn't come from outside factors alone (the love of the heroine) but also from within (his own willing heart), I can believe that even if something happens to the heroine in the future, he won't go back to his evil ways. I can believe love has truly made him a hero, and that he'll stay a hero, no matter what.