Perhaps it should be born in mind that political violence is likely to decay into mere criminal violence. The Mafia and the Irish Republican Army were once political organizations, and both degenerated into mere criminal gangs.
And as someone else has already pointed out, political violence has a way of backfiring. Terrorist violence against the civil rights movement backfired very badly. The biggest problem the insurgents in Iraq have is that their own violence has badly alienated many of the very people whose support they so badly need.
And once you’ve let that genie out of its bottle, it can be awfully hard to put it back in; it becomes easier and easier to rationalize violence every time someone gets in your way. Communist terrorism in Tsarist Russia ended in Stalinist repression that killed tens of millions of people.