We had an incident near me a few years back, where a girl whose car was broken down on the highway was kidnapped, raped, and murdered by a man pretending to be a plainclothes cop.
If the cop is not in uniform, and his vehicle does not have blue and red cop-lights, then he should be forbidden to approach motorists, who can quite reasonably think he's a robber.
That's the bottom line right there! This citizen's death was caused by the cop's negligent actions. The citizen's family should file a huge lawsuit. And if I were the DA, I'd look into charging the cop.