Seeing a villain get a redemption arc and turn into a hero is always fun.
But honestly...I kind of prefer seeing the hero turn into the villain.
A lot of people would claim Walter White from Breaking Bad is the best example of this. We see him go from family man and chemistry teacher to ruthless drug lord.
Others will claim Anakin Skywalker from the Star Wars prequels had the best descent into villainy of all time.
Although Cady from Mean Girls could give him a run for his money.
Or maybe you prefer more subtle arcs, like Lena Luthor from Supergirl becoming more villainous after feeling betrayed by Kara.
