When someone knows that you are in your home and they break down the door,it has to be assumed that they do not mind hurting or killing you.
In that case firing 2 or 3 times is definitely called for, shoot until the yelling stops.
Some years ago I took some private self-defense lessons from an NRA instructor. One of the things he told me was that you continue shooting until the perp drops to the ground. Then absolutely no more since if you do, the justice system will nail your hide to the wall.
Here, the homeowner could not see what was going on behind the door but I think he should have fired more than once to be sure the threat was neutralized.
Idiot probably knew an old man lived there and chose the house accordingly.
But, once a marine, always a marine.