Two possibilities.
1: For the celebrity. Think about it, if you were a legit supervillian in modern society, you could market your image all over the place.. just as long as you didn't seriously hurt too many people, and had a good enough PR team.
2: More likely supervilliany would stem from an insatiable desire to prove that you are as amazing as you know that you are to the world. Whether it's harassing the world's most beloved defender, just to prove that you can, and are therefore better than him, and therefore the world, or stalking/kidnapping attractive/successful people and torturing them in your lair until one by one, the best of the best admit that you are a better specimen of humanity than them, right before you kill them. In reality, it's all about proving you're better than the common man. Comquering the world isn't as important as proving to the world that you deserve to rule them (regardless of whether or not you actually do rule them.) If you want money and women, being a superhero is the clear rout (as long as you have an image that can be merchandised.) If you want to help the greater good at all costs, again, you're still pretty much a superhero, even if your behaviors seem villainous. Sure it'd be nice to be ruler of the world... but wouldn't it be better to prove to the entire world that you SHOULD be their ruler (and then deny them the courtesy) ... because you know that you should... and the fact that they don't agree proves that they don't deserve your benevolent rule... and if you don't KNOW, then you're not a real supervillian... not yet.