Reminder: If that's a shinigami he shouldn't be needing blood transfusions.
Based on the knife, this is the guy who killed Agni. What likely happened is that Polaris and Ciel showed up at the door together, and Polaris did the dirty work. I don't think Ciel is physically capable of beating the shit out of Agni if he's just a normal zombie, since he just doesn't have the muscles.