You may be looking for:
- Fair-Weather Friend: They are your friend when things go well, but not when things go awry.
- False Friend: They pretend that they're your friend, but they're not.
- Psycho Sidekick: A buddy of a morally-upright protagonist who is not as squeaky-clean.
- Psycho Supporter: A madman who as a result follows someone's beliefs.
- Sparing Them the Dirty Work: Someone (usually a friend) who exacts revenge on someone else's behalf to keep the latter's hands clean.
- Toxic Friend Influence: A friend who negatively influences other friends.
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