How about "He who lives by the sword, dies by the sword"?
The Catholic tradition condones war only if it is a "just war". What we are doing in Iraq is not "just". In fact, I'd be hard pressed to find any war a "just war". Here's more on it:
http://www.americancatholic.org/News/JustWar/Iraq/
Deciding whether or not to kill a terrorist is one of those apples vs orange choices. Neither is good building material. When presented with it, I choose cinder blocks.
The simplistic answer is to say, "well, he's got a gun, and he's totally unreasonable, so I have to kill him". The hard answer is to spend a lifetime promoting human dignity so that terrorism cannot take seed and grow. When we promote universal literacy, civil rights, and health care, teach people non-violent conflict resolution, and listen to and enfranchise those who are disenfranchised, we decrease the chance that those people will seek extremism and violence as the only perceived solution to their plight.
An example I heard recently was this: Should a professor keep a gun in his drawer in the class room, just in case a crazed person walks into a classroom and threatens to shoot everyone? Would shooting the crazed person be the right thing to do? It certainly seems simple enough. But how many hundreds of people are not turning into crazed gun-wielders right now because their teachers, parents, clergy, doctors, friends, co-workers, counselors, and people they meet on the street are helping them to not go down that path?
You cannot know what ripple effect you are having, when you help a friend through a rough day, and she in turn does not yell at her kid, and he in turn does not feel more isolated and angry. Just one act doesn't make much difference in the world (unless it is shooting a kid). But thousands of acts in a lifetime, by millions of people, can make this a better place.
I challenge myself to "fight the good fight" by waging peace daily.
In the midst of winter, I found there was, within me, an invincible summer. ~ Albert Camus