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. Unlike in Happy Ending, this turns out to be a bad thing as Villain is to The "Villain" as Hero is to The "Hero". Villain drops some sort of powerful thing for Hero to obtain (i.e some sort of energy, weapon, or tool) that they then use to accomplish their own surprising evil bidding. As this thing did originally belong to Villain, it's identified as evil, as well as its wielder. Some time later, Hero becomes the new Villain and does bad stuff because that's what their entire adventure was actually about.