In my small experience, this compassion for those who cause harm is more of an experience than a thought or pattern of thinking. Or I'd call it a habit pattern. The compassion arises or it doesn't. Like you get a belly ache after a big meal or you don't. For as long as it doesn't arise, there's practical things you can do, like I mentioned in my previous post. For example when I watched those crime shows, the experience of compassion was spontaneous and there was no thought proceeding it that I'm aware of. It was the subtle thought hiding somewhere in my mind that due to this action, much further suffering would come about. But that's often in the back of my mind, maybe because I seem to attract difficult people on a regular basis? So compassion was the result while I was receiving that information.
Another thing is that I had this experience once where a client who I really cared about murdered someone. That's a whole other story. But it really brought it to a personal level and I had no choice but to face this issue.
Have you ever done metta meditation? It's an interesting experience