" He was going to kill you,” she said. “And me. Fair’s fair. An eye for an eye, a tooth for a tooth, a candy for a candy, a penny for your thoughts.” “Still and all. We don’t kill unless we have to.” River reflected on this, then smiled brightly. “Okay! That’s a good rule.” “I like to think so. "

― James Lovegrove , Big Damn Hero (Firefly #1)