You may be looking for:

* FairWeatherFriend: They are your friend when things go well, but not when things go awry.
* FalseFriend: They pretend that they're your friend, but they're not.
* PsychoSidekick: A buddy of a morally-upright protagonist who is not as squeaky-clean.
* PsychoSupporter: A madman who as a result follows someone's beliefs.
* SparingThemTheDirtyWork: Someone (usually a friend) who exacts revenge on someone else's behalf to keep the latter's hands clean.
* ToxicFriendInfluence: A friend who negatively influences other friends.

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