Idea brainstorming: can you suggest some motives for a criminal to kill cops?

Just a little help for my brainstorming process. I’m a bit stuck in the middle of my story and I need some fresh ideas to go on.

Here’s a little summary of the story: the time is the 1980s. Someone is killing cops and a maverick cop is trying to find the culprit. The cop had his cop father killed apparently by the same killer. The case gets personal when he finds out that his partner (and love interest) is going to be the next victim. Why do you think the killer is after those cops?

Just suggest me some ideas to help my mind go on. It will help me a lot.