From what I heard, some of it is personal and some of it is just random violence now. He certainly controls the police so they'll kill whoever he wants them to, but the general atmosphere of police lawlessness that he has instituted also allows the police to go around doing what they want and killing who they want, knowing they won't be held accountable.
This is in a country that ALREADY had police so violence they'd put American police to shame. Another friend I have there said back in the late 1990s that several teenagers from the slums got arrested on baseless charges without warrants, so two of his friends went to the police station to argue for their release. At the police station the police refused to release the teenagers, refused to state what crimes they were arrested for, and when his friends complained, the police shot and killed one of them right there in the police station, then told the other guy that they'd kill him too if he told anyone. The other guy went into hiding soon after he got back because he was afraid the police would think they made a mistake by letting him live, and my friend never heard from him again. He doesn't know if maybe the police did get him too in the end.
That shyt was already going on, but Duterte just makes it a free-for-all to know any cop can do any of it without even a chance of repercussions.