Many comic villains flit between doing good and evil as if they're options on life's great buffet. While it's endlessly interesting having to guess whether a character will do the right thing, this pales in comparison to one of the most interesting storylines a villain can have - namely, their reformation.
And a surprising number of villains have made this storyline last for good, making their reformation permanent. From heroes who turned good so long ago that you forget they were even evil - like Hawkeye and Plastic Man - to characters like Tomorrow Woman who exist solely to provide an interesting villain reformation, there are endless ways that this kind of storyline can be done, and all of them are equally fascinating.
As much as it's fun to see villains be absolutely terrible, there's something about watching a bad guy redeem themselves that just makes you feel good.
Not only does it make for more three-dimensional and ultimately more human characters, it also comes as a solid reminder that you don't have to be great 100% of the time to become a good person - you just have to want to do good.