Oh, I'd say there are plenty of times where killing another human being is justified--and OP's scenario here is one of those IMO. 

Some states in the US have "castle law" where you can kill a burglar with no legal repercussions if you think your life is in danger (or not).  Other states say you have a duty to retreat.  I like the castle law philosophy, though I've never killed anyone.  I would imagine I would not feel like a more complete human being for having done so.