You can only try to help those who have true beliefs better publicize them.

It's true that if the truth of a belief were sufficient to spread it, it wouldn't matter if one were nice or polite. However, I believe that, as a long term strategy, mockery is not the best way to spread beliefs (though it might be a good way to destroy them), because no matter how right one happens to be, people don't want to listen to a dismissive douchebag. Satire, sarcasm and irony are therefore weapons to be wielded with rather more care and precision than I have seen most of their users display.

Therefore, you might have the ethical injunction to never, ever respond to a bad argument with a bullet but might permit yourself to make your verbal counterargument more punchy.

It certainly is a matter of orders of magnitude. There's a point from which quantitative changes take a qualitative character, and causing death is one such point.