A week of bounty hunting at a nav point. I use the police to help me take down targets. Not once have i hit a cop. Not once have the cops turned unfriendly on me. Care and attention. Situational awareness. And check your fire.
I know the arguments "but the cops flew in front on me" and "its immersion breaking" - to be honest, for me this is perfectly immersive. You are the mercenary here, getting in the way of the cops. They should have no reason to give you any slack. Stay out of their way. If you want to help taking down a wanted person, you do so at your own risk. Mess with the cops, they will mess with you right back.
I have heard about cops going red without reason. I've not experienced that myself. Either there is a bug here (and it has been reported, so if it is, it will be fixed), or else players are just not realizing they are causing the NPC to go red for some reason.