The reasoning behind your evil villain (their motivations, their past, and the environment surrounding them) is what is most important, when considering where to take their actions and how to have them grow as a character.
For instance, my primary villain is committing heinous crime after heinous crime, twisting the subconscious nightmares of her victims into real entities that attack, kill, devour, etc. human beings in the conscious world. Yet in doing so, she's actually helping to push back something worse: a threat that would quite literally cause the End of Everything if she were not taking the actions she does throughout the story. The main character, or 'hero', then has to decide whether the considerable amount of 'help' this is affording to that end is worth the disaster in the meantime, and try to find an alternate method of doing things that won't result in everything being returned to nothingness.
My advice is this: the best Villains, and it is just my opinion, take actions that serve to set up a series of choices, which ultimately cause your good guy to be conflicted about what to do next. They're there to challenge the protagonist, to try to make them see things a different way, even if that way is absolutely terrifying or nauseating in terms of morality.
