Here is Jasper's perspective from New Moon p. 300.
"I traveled with Peter and Charlotte for a few years getting the feel of this new, more peaceful world. But the depression didn't fade. I didn't understand what was wrong with me, until Peter noticed that it was always worse after I'd hunted.
"I contemplated that. In so many years of slaughter and carnage, I'd lost nealy all of my humanity. I was undeniably a nightmare, a monster of the grisliest kind. Yet each time I found another human victim, I would feel a faint prick of remembrance for that other life. Watching their eyes widen in wonder at my beauty, I could see Maria and the others in my head, what they had looked like to me the last night that I was Jasper Whitlock. It was stronger for me -- this borrowed memory -- than it was for anyone else, because I could feel everthing my prey was feeling. And I lived their emotions as I killed them.
"You've experienced the way I can manipulate the emotions around myself, Bella, but I wonder if you realize how the feelings in a room affect me. I live every day in a climate of emotion. For the first century of my life, I lived in a world of bloodthirsty vengeance. Hate was my constant companion. It eased some when I left Maria, but I still had to feel the horror and fear of my prey.
"It began to be too much.
"The depression got worse, and I wandered away from Peter and Charlotte. Civilized as they were, they didn't feel the same aversion I was beginning to feel. They only wanted peace from the fight. I was so wearied by killing -- killing anyone, even mere humans."
I had forgotten till I looked this up just how much Jasper's victims' emotions affected him. I'd be willing to bet that eventually, he might have discovered a "conscience," too--just as the Denali vampires had.
"Where there is great love, there are always miracles."