1. Daryl. The most obvious possibility, and probably who the show wants you to think, which means it's probably not. Daryl and Justin came to blows twice in the episode, and when Rick separates them during their second fight, Daryl says, "There's only one way to deal with these assh---s." We might be witnessing what Daryl meant by that. Daryl and Rick are not seeing eye-to-eye on how to handle the Saviors, and Daryl might be implementing his own method of Savior management. Which is assassination.