That's not completely true though, a large part of police work is also preventing violence against citizens. What would the robot do if it arrived at a domestic violence call and needed to save a women/child from abuse? I could see a massive public outcry if the robot simply watched the crime take place.
It would disarm and detain the assailant. The same thing a human police office ought to do in that situation. Are you trying to argue that it should just kill the abuser?
Certainly not in every case but there are a lot of times when disarming is not an option. I don't know if you have had any kind of defence training but I have and disarming an armed opponent is one of the hardest things to train for. More often than any one would like the police really have no choice.
