I'm going through a similar situation, bittersweet.
I don't think you "shouldn't care." There is nothing wrong with feeling compassion for someone you once loved, but his choices are his choices. He didn't have to self-destruct just because you made the decision to leave.
Keep on the straight and narrow path. It sounds like you're doing very well for yourself. And you may want to consider going no contact to avoid these feelings in the future.