The best villains are ones who don't want to be villains. One of the best examples of this for the Teen Titans is Plasmus. Why? Because he's a kid that has a symbiote that lives inside of him, and it manifests when he's awake. Thus, he has to be kept in a constant state of sleep.
Naturally...that doesn't happen all the time. Slade (among others) continually wake him up, and unleash Plasmus onto the world, and thus the Titans have to stop him.
Interestingly enough, Plasmus is one of the few villains to actually evolve physically through the show. He once was thrown into a pool of toxic waste, which mutated him even further, and thus made him and even more dangerous threat.