Here's why you don't frikkin' throw bombs.* Made a right damned mess, didn't it? Empowered a lot of ijits, didn't it? And it was one of the first demonstrations of the now-old saw that in any movement, the informer is prolly the guy urging you to blow stuff up. (The preceding link is wrong; I probably intended this one).
Morally, one may respond to force with force. One does not initiate force.
(* 24 Aug 2010: This is totally the wrong link.