I think at certain points throughout Angel, Lorne was taken for granted a lot. He was put in a lot of positions he didn't want to be in because of his ability. I felt really sorry for him, especially when he shot Lindsey. One can see why he had had enough. That, and when Lorne told Angel to never look for him again, were very stinging. Especially since Lorne was used a lot and it seemed like he was very under appreciated.... Not to mention, he never wanted to be violent in the first place.