Fan Casting Abandonment-Induced Animosity in Sort by Tropes

This trope is when the villains or heroes leave a member of their party behind (either intentionally or not), and the abandoned turns on them as a result. Maybe The Dragon or the Big Bad managed to convince them to switch sides using this reason. Maybe the abandoned doesn't change sides but simply decides that they want to get even. Maybe the villain doing this to one of their mooks causes them to join the heroes in retaliation. If/when the abandoned sees their former allies again, expect a "The Reason You Suck" Speech directed at the abandoners.