Sudden Corruption, Hero Malfunction

Just at the end of The Big Fight, Hero wields their weapon, glares at Villain, and slaughters them right on the spot. Very much unlike in Happy Ending, this turns out to be a bad thing as Villain is to The "Villain" as Hero is to The "Hero". Hero, err, "Hero" was actually going after Villain to obtain some weapon or power they held and they were using their allies as tools. Or something like that. Most of the time, someone snaps Hero out of it and rescues them from the dark side before Hero could do anything too evil.