Normal set of definitions:
Polemicist: A person engaged in or inclined to controversy, argument, or refutation.
Troll: One who posts a deliberately provocative message to an online board with the intention of causing maximum disruption and argument.
Alternative set of definitions:
Polemicist: One who informs the world of where it is going wrong and uncovers this wrongness wherever he finds it.
Troll: Anyone who deviates from the message of the polemicist even slightly.
Again, Hmm …