Most anime protagonists are decent folks. Maybe they make some mistakes or bad decisions, but they mean well. While these kinds of main characters are common, they're not universal. Some main characters are cold-hearted, cruel villains.
Anime with villain protagonists allow viewers to adopt a perspective outside the norm. These anime ask viewers to empathize with characters who torment and seek to ruin others. This can be a disconcerting experience, but it can also be a great mental exercise. Maybe the character is hellbent on world domination, maybe they're seeking revenge, or maybe they just don't know how to handle a failed relationship. No matter what, they're doing something wrong, and it's the antagonist's job to try and stop them. Sometimes, these evil protagonists can even make for the best villains of all time.
Vote up the villainous main character who's compelling enough to get you rooting for them... or at least invested in their story.