Hey by, I think yomintz is looking for a noun that incorporates what you've suggested, but without the malice aforethought - someone who has this ill effect unintentionally, perhaps. "Well-meaning snake-in-the-grass" doesn't have quite the right ring to it....Isn't yomintz looking for a noun to describe someone who brings strife but doesn't realize it, and is then surprised and hurt by the reaction he causes? Bill's attempt with "catalyst" is good, but I think there must be a word or phrase that's in more common use....Perhaps something incorporating the word "innocent"?

PS to Bill: I DON'T have macular degeneration and I also thought, at first, that the handle was "vomintz"!

Let us go in peace to love and serve the board.