Psychopathy, combined with obsession. Neither one are among my personality traits in real life, but, if I had to be a supervillain, that would be my preferred type. I wouldn't exist to take over the world, I would just be completely detached from all other human beings, and live a chaotic existence in the absence of empathy. I'd also function by getting into people's heads, exploiting their weaknesses and trying to break them.
I'd be one of those supervillains where my primary motivation is always really unclear, aside from the fact that it revolves around the superhero, and trying to surpass them. I would fight against other supervillains if their goals conflicted with my own. That would even include rescuing the superhero at times because I would want to be the one to who gets into their head and destroys them.