@Ragman,
Thanks everyone. Provocateur seems closest, but not on the mark. A provocateur doesn't always set one group against another for their mutual destruction.
There was a Japanese movie staring Mifune Toshiro, in which he played a ronin (masterless samurai) who joined two opposing gangs, each thinking he was a double-agent working on their own behalf, and he expertly set both sides against each other. This led to an all-out gang war, during which he mostly just sat by and watch both sides annihilate each other. He did some fighting but mostly, the gangs accomplished the desired action of killing each other off, leaving the town free of both gangs of ruthless criminals. Mission accomplished, the ronin left town at the end of the movie. Much like a Clint Eastwood movie...
I'd like to know what the term is that applies to that kind of person. Sometimes he does this for a good or moral cause, sometimes he just wants to see destruction.