Another fun thing to do with your villains

Here's something I forgot to mention in my post about villains that is completely optional, but would make them really interesting.

Make them right.

I don't mean "make their actions completely excusable and secretly they were the protagonist the whole time." I instead mean to make their philosophies right, they just went about the wrong way to prove these philosophies, or they did wrong things in the name of their right philosophies.

Ideally, after the villain is confronted and stopped from whatever they were going to do, the heroes should realize that while the villains actions were wrong, their ideas were not. And perhaps it makes the heroes really think a lot about who they are and what they're doing with their life, and maybe they gain a new quest objective after defeating the villain. (They don't necessarily have to go through with said new objective in the story, it can just be implied that they will go and do this after the story's close, or perhaps it's enough to make a sequel.)

A good villain makes not only the reader, but the protagonists rethink major ideas they once had.
A good villain teaches that the difference between "good guys" and "bad guys" isn't that one is right and the other is wrong, rather that one is willing to do whatever it takes to achieve their goals, no matter who or what gets hurt in the process, and the other is going to find a less destructive means to the same end.