While I absolutely love this episode for Piper's power, and the faking of the documents and the office scene.
Like others, I have to ask the question: why the hell would they automatically believe he wants his powers stripped so he can use the amulets? Sure, it is not an implausible plan/theory, but it hardly fits the facts better than the truth... you are witches, you know about demons and magic and spells and possession but you conveniently don't think of any of that here?
Secondly, once it's done, ok all you saw was him killing a woman, be angry about that, but why not let him take the potion? God, why would you let that demon who is apparently all evil and scary and able to get through amulets, allowed back out on the streets WITH his powers? I'd rather hate someone AND have them mortal, than hate someone AND them be able to come back and kill me if he is truly evil.
I thought the death was a bit tedious, like someone else said, if Piper can't vanquish Cole with her powers why then his 'brother', we established they're blood connected, and one vanquish potion can work for them all. Maybe this guy wasn't a fully fledged brother then I don't know.
Not the worst, not the best. And it is hardly evil to kill a manipulative high ranking demon.