Dealing with the necessities of the real world means drawing lines, and just because it may be ridiculous to draw the line in one place does not mean that the whole exercise is ridiculous. At the opposite extreme: even if a terrorist is heading at you with an AK-47, you shouldn't "commit an act of violence" because what if he chances his mind and decides not to shoot you?
That is comparing indiscriminately violence to self defense against a specific identified person.
The lines are not that silly. Yes, you can defend yourself against someone pointing a gun at you. No, you can not use a nuke, or release biological weapons that kill indiscriminately everyone in the whole nation. Reactions should be appropriate to the threat, and indiscriminately violence is rarely if ever appropriate.
It's a "reductio ad absurdum" argument. You know, test for the extreme values that can be passed into a function to see if that function can handle them. If it can handle the extremes, and a few other values correctly it probably handles everything correctly.
