"We will not allow her to fall from grace. Instead, we will be the sinners."
— From Afro Samurai: Resurrection
A character defined both by their obsessive loyalty toward a target character and by their vastly divergent (and comparatively "cynical") moral code.
The Poisonous Friend is usually attached to a pacifist protagonist or an idealistic villain. They tend to consider their "master" a person to be worshipped or protected at all costs — even ones that the master would not knowingly permit.
For example, if the master is a Fettered pacifist completely against killing his enemies, the Poisonous Friend might pay lip-service to this trait while the master is nearby... and then turn around and butcher the enemy behind the master's back. If caught, they'll claim they just "did what had to be done". Few masters catch on.
Sometimes the master's influence gets through to the Poisonous Friend, with results such as Badass Decay. In other cases, the Poisonous Friend continues to pose a danger both to the master's other friends (through jealousy) and to the enemy. The Love Interest is in particular danger of the Poisonous Friend if they suspect them of making the master "vulnerable".
Though sometimes introduced as anti-heroes or turncoats from the other side, they cease to be those things because they place their fundamental loyalty upon the shoulders of their "master". Basically they become a good guy's secretly-evil minion, or a bad guy's much-badder sidekick.
This character is often used when writers don't want to stain the "innocent" master's hands but still need a way to deal with those pesky defeated enemies. Even friends who have been explicitly told to behave or otherwise decided to hold back can still function in this regard if they have done something bad or really bad enough to intimidate the villains. If a particularly noble and idealistic hero has a particularly cruel and ruthless poisonous friend, the villain may hesitate to take advantage of that hero's idealism and kill him out of fear of what his buddy's response could be.