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I'll kill a man in a fair fight, or if I think he's gonna start a fair fight... If he bothers me, or if there's a woman... Or if I'm gettin' paid. Mostly only when I'm gettin' paid.
— Jayne · Serenity

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker begins with a fair fight as a reason for killing, then keeps adding less principled exceptions. The final emphasis on payment undercuts the earlier appearance of a code. The humour comes from a supposedly careful distinction dissolving into a blunt admission about money.
Context reviewed