One of the things I like in both characters and plots is the theme of redemption: a wicked character realizes he's been doing evil things (possibly after the hero beats him up, a lá Dragonball Z), repents of his evil deeds, and turns to the good--or at least, not murderously evil. It's a theme that strikes a chord: we're none of us so evil that we can't get our acts together, be forgiven, and turn ourselves around. Ditto for our neighbor. It's one of the fundamental teachings of Christianity (or Buddhism) and some other religions as well.
It's also a consequence of writing three-dimensional antagonists: if they have motivations other than "I'm a raging sociopath who likes to hurt people", they often wind up looking more tragically misguided than villainous. The thing about being misguided? You can be pointed back in the right direction; a well-written character will suggest that possibility to the reader/viewer. It then becomes tragedy if the character persists in his downward path, or a dramatic redemption if he finally straightens up.
Thanks
1 to the lingering effects of the Hayes Code and the Comics Code, that was a very rare theme in drama when I was growing up. It was so rare, I cherished every instance of it. Thanks to the Hayes & Comics Codes, generations of Americans were taught the frankly unholy belief that people were either Good or EVIL, Good people were GOOD all the way through, and EVIL people were irredeemable,
unforgivable, and deserved misery and death. In the rare instances that somewhat evil people turned good, they always had to die to make up for their crimes, though they might get to die heroically. (See
Redemption Equals Death). One could argue that the inculcation of such beliefs has unfortunate real-life consequences, such as the notion that "criminals" deserve no mercy and should be tossed into jail to rot, and never given a break even after their sentences are up, or that people who imbibe officially-disapproved substances are horrible criminals who deserve never to hold an honest job again.
Thus I cherish characters such as Marc C. Duquesne in the
Skylark series (though he more decided there was no point in being a moustache-twirling villain when he had his girlfriend and the entire universe to roam around in than actually turned good), his possible homage Marc Remillard in the Galactic Milieu/Pliocene Exile series (who was definitely redeemed), and, of course, Vegeta in DBZ. If you consider the entire Dragonball Z series, one could argue that the fundamental theme is the redemption of Piccolo (at the hands of Gohan) and Vegeta (at the hands of Goku).
This probably explains why the overarching theme of my DBZ fanfiction is the redemption of Raditz. It's probably why I like Decepticon-to-Autobot defectors in Transformers: I like characters who realized they were doing evil and turned away from it, and were forgiven.
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1 And I mean that in the most sarcastic tone possible.