"Besides, I already made you a promise."
You're on your knees, and the guy saying that doesn't smile. It's not a good situation. It is, however, cathartic for the audience when the guy on his knees is a smug young cannibal whose luck has just run out.
So what was that promise anyway? Let's see... Oh, yeah. Back in the season premiere, when asked to detail his litany of weapons for said smug young cannibal, Rick concluded thus: "...and a machete with a red handle. That's what I'm going to use to kill you."