I played a really evil villain in a faction of villains - he stood out, you know? - and had a blast. We'd plot bad things to do and then go do them. We didn't even try to PK every time, knowing sometimes the best victory is one where you leave your opponent alive with the knowledge you took everything else from him.
In the end, there was a chance for my PC to go out with a bang at the climax of an internal power struggle. It was one of the best arcs I've ever been a part of.
Being the antagonist, in my opinion, is way more fun, especially if you can work with the protagonists to create something truly remarkable.