Ideas for "Grey morality" optional side quests

[Alexander Atoz]

I do have a question about them though. What makes the villains... villains?

I was thinking of just going with a monster type that is generally accepted to always be evil (but not the type of evil that would set out to corrupt people, like devils and hags, as that would immediately lead the game in a different direction.) I was thinking green dragon, but lich, mind flayer, beholder (I think they're still considered evil, despite Xanther's), and doubtless many others should work.
(My examples are from D&D, which is the RPG I know, but I would assume that all/most RPGs with a monster manual would have monsters that could be used for this.)

And thank you very much for the story. I was curious.