Remember the larger goal, which is to find out where he came from, who sent him. Killing him accomplishes neither of those. I don't have any strong feelings about exactly what you should do at this point, but the first thing that pops into my head is… run like a little girl. Of course .... <gesturing to her own diminuativeness> ... you know.
Now, that plan has its own dangers. We definitely don't want him getting away. Even killing him seems better than that. You could call for help, and fight defensively in the meantime, which would mean you take -10 to your initiative and add your willpower to your defense roll for the entire turn. If you go that route, you should do it next turn, not this one. I think he's done attacking this turn.
Or, you could do something entirely different. The only thing I feel very strongly about is that you should remember the goal here.