This is a sneaky question, at first glance it looks like a well-formed third conditional. The trap is in the word 'now' at the end! This means we should be looking at a mixed conditional where the first part is from a third conditional talking about something in the past but the second part (the result clause) is from a second conditional talking about the present. So the correct version should be: "If the bullet had been one centimetre to the left, he would be dead now."
Hope this helps