I've played an evil "plant" character in collusion with the DM before, but it is important when that happens, I think, to understand that your role as the evil player is to betray them dramatically but for the heroes to still win. I would have felt really guilty if my betrayal had *actually* killed the party. (As it happens I died in a monster encounter before we ever got to have the big reveal.) Now, killing one or two of the other people as part of the final betrayal makes for good drama.
That's really the only situation I'm comfortable with evil characters when I myself am the DM - just one, and he has to be on good behavior when the rest of the party is there, and ultimately the character is probably doomed. There are only certain players I would trust to pull this role off well, too. Anyone who is inclined to tend towards BoVD behavior with an evil character is out of the question. (That's what NPCs are for, I'm really uncomfortable with the idea of not controlling the exact amount of that sort of content in my games. It can be used, sparingly, to shock the characters into action, but should never become commonplace, lest it become mundane.)