I find that you can't really engage other players as a villain without a plot already established. Unless the interaction is integral to the villain's goals, it winds up being no different than normal. Try not to approach the RP with the intent to do wrong; instead, just be true to the character and remind yourself of what they want, then turn to morally questionable methods to get the goal. But again, if the interaction has nothing to do with those goals, there is little reason for your character to misbehave. If you really want to be an antagonist, you need a plot.
