Equilibrium (Balance) is a natural state in our universe.
It exists between Bright and Dark, Order and Chaos. What we call "good" and "evil" are extreme symptoms on the spectrum of being unbalanced. Being completely self-serving can be as spiritually unhealthy as being completely self-sacrificing can be physically unhealthy. See how that complementary thing works? Good. Now, what does it mean for your antagonists and protagonists?
Superman and certain other superheroes can get away more altruism than most because of their powers. For most mere mortals, though, not taking care of yourself comes with great consequences. A hero can only donate so much blood and flesh, block so many bullets and knife blades, and chase societies enemies for only a finite amount of time without some rest and a bite to eat. No, all the food can't go to the starving children. You have to take care of yourself to a certain degree to be able to continue making sacrifices for others.
Sliding to the other end of the spectrum, kicking puppies and eating orphans will damage the spirit and erode the villainous from within. Vampires lose their reflections and we could go through a litany of specific vile figures whose dark indulgences have guided their withered souls far from the light, but I'd be naming politicians and supervillains all day. Their cost is more than just white hair and disfigurement, it's the essence of their humanity itself. Each destructive choice takes its toll on the villain as well as the victim. At the extreme, what remains will be either a figure of unfeeling cruelty or utter madness with neither maintaining more than a parasitic or predatory connection to mankind. And we're back to Congress again.
Let me get to that divergence another time. The point is that when you develop your characters, there's a lot of middle ground available between Paragon of Virtue and Embodiment of Malevolence. I'm sure you've heard it before, but many of the most interesting characters are going to be found occupying those middle grounds. In the course of going through a story, they may even change positions on the spectrum. It doesn't have to be a change from one end to the other. The Grinch managed to pull that one off, but not everyone can do it as convincingly. What's important is that the audience gets to learn more about them by the end than was known at the beginning.
If you're feeling generous, you might even let your character in on it.