No, not really.
I highly doubt that anyone is going to be such a fucking moron that they would really intentionally try to destroy the planet. Hell, even life in general. Life, once it starts somewhere is pretty hard to get rid of. Single species come and go often, but life in general won't just roll over and die.
Also, we can't be the biggest threat to humanity, because if humans didn't exist we WOULDN'T EXIST. Neither would Global Security, by the way. :D
EDIT: Oh, and what Blacken said, although he said it better I think. :(