Don't. Mess. With. The Grimes. Adding to the whole notion that his group are building a reputation, Rick gave a first hand demonstration of what he's capable of, even in dire circumstances. With a knife to his own throat, he managed to shank a guy in the neck, before a great moment of standing up and turning around to see a shocked crowd. "What...?" Rick asks, with a confused face, dripping with blood. It shows how massively desensitised Rick has become when he - and his group - are threatened. It was also a gory reminder that nowhere is spotless, and even behind the walls of Hilltop, bad things happen.