First of all, I'd like to thank you all for responding to what I have to admit is an unrealistic scenario. In real life, most intruders are motivated by greed and the thrill of criminal "adventure", not bloodlust, and so they wouldn't make a noisy, frontal assault against an armed homeowner.
Next, before addressing the legal considerations of this scenario, I'd like to get some agreement on the tactical side of things. Doesn't knowing where the intruder is (right outside the door) while he has only a vague idea of where you are (somewhere behind the door) give you a definite advantage that will lost once the door is down? Once the intruder enters the room, you can ID him (provided that there's enough light) and know exactly where he is, but he also becomes a moving, shooting target with a much better idea of where you are (depending upon the lighting). He might also have an accomplice right behind him, in which case you now have two targets to deal with instead of one. It seems to me that shooting through the door would be an application of the military principle that "all obstacles are covered by fire". (BTW, I still remember being put on the spot by one of my drill instructors with the question, "All obstacles are covered with WHAT?" My response: "Barbed wire, sir!" Oops, wrong answer.
Anyway, I also think that in this scenario you can, perhaps, "identify" a target without visual contact. Consider the three requirements that someone must meet to qualify as posing an immediate threat of death or grave bodily harm:
1) Ability - Yep, you've already heard gunshots, so you know the intruders are armed.
2) Opportunity - This one is a little sticky. You can't see if the weapon is actually pointed at you. Then again, you're well within range and the intruder is one good kick away from having a clear shot at you.
3) Jeopardy - These people have already broken down your front door and killed your dog. They probably aren't here to sell you Girl Scout cookies.
You also have a good idea of where the target is, so while you will be firing "blindly" in one sense, your odds of a hit are pretty good. It's also possible that you can end the fight with one shot (killing/wounding the kicker and prompting his accomplices to flee) rather than risking a hail of bullets once the door is down.
Let's also consider two variations on the original scenario:
A) In this case, after the initial jiggle of the doorknob, the intruder outside the door starts shooting through it, rather than kicking it. Does this change your plans?
B) In this case, you've recently provided testimony which has resulted in the conviction of a local criminal on multiple felonies. You learned at the trial that this felon belongs to a violent, drug-dealing gang, one of whose members said to you, outside the courtroom, "You're a dead man". Now the intruder who's running up the stairs is screaming something along the lines of, "Hey, so-and-so, it's payback time!". Does this change your plans?