Source: boundingintocomics.com

1. Phoenix/Dark Phoenix

(Uncanny X-Men, The Dark Phoenix Saga, Marvel Comics

This one had to be here, but its circumstances are much weirder than they initially seemed to be. In the original comics, Jean Grey became the Phoenix when cosmic energy upgraded her powers. Some time later, she snapped as she dealt with the Hellfire Club (mostly due to some really gross moves on Mastermind’s part) and became the Dark Phoenix. She ate a star, killed billions, and was sentenced to death.

The X-Men fought on Jean’s behalf, but she eventually took her own life to avoid being out of control for the rest of her life. Buuuuuuuut none of that actually happened. You see, due to a massive retcon, it turned out that Phoenix was a completely separate being who posed as Jean. Phoenix turned evil and died, but Jean didn’t. Instead, she spent a few years in a cocoon floating in a bay.

And there you go: a list of ten hero-to-villain transitions, all of which deserved a mention. Maybe I'll do the other half of the exchange next time.