A troll is someone that gains value from the act of causing others to lose value.
Okay, that was a bit dry sounding. How is this: a troll is someone that takes pride and joy at causing misery, anger, and distress in others; a troll derives pleasure by actively creating displeasure in others. However, a troll isn’t just schadenfreude; a troll gains value by being the direct cause of that schadenfreude. Thus, a troll is comprised of two major parts: emotional schadenfreude and personal responsibility.