The proof would have to be in your ability to prove that he was a continuing threat at a level that a normal person would agree. In this case that would be difficult. Perhaps if he had yelled something threatening or was calling for backup from an accomplis outside, you might have a case.
But the key here is that the protections of castle would fail you in the sense that the assumed innocence would move to assumed guilt. You would be on the defense and in order to stay out of jail you had better shoot the moon and play every card right.
On the other hand, assuredly you had a reason that you took the shot. That reason is the beginning and end.
If the reason was your move and shoot training and you just kept engaging targets - welcome to manslaughter.
If the reason was your rage over the attack - welcome to manslaughter.
If the reason was your property - welcome to manslaughter or worse.
If the reason was your fear of him - then a case still exists but you are in for a legal ride. Look at your home now, you'll probably be selling it for legal fees later.
If the reason was a direct, understandable threat, also recorded on your film, like calling for aid from a third attacker outside - then you may be ok and might keep the house.
I will add, however, that in any of these cases, the most important thing is that your family is safe, even if you are not.
Not a lawyer, but spent plenty of time in harms way. I have to go with BG1, dead before he hit the ground. His cowardly, sniveling, buddy, BG2, as much as I would want to take him out of the breeding pool, I have always been told, No threat, No shot. If he is headed towards the door at a high rate of speed and is no IMMEDIATE danger to me or mine, perhaps a bulldog take down and some uncomfortable restraints until the LEOs arrive. I think in this scenrio, I would take my chances with a potenial charge of holding him against his will vs manslaughter. LEOs would certainly charge him accordingly and take him off the streets for some period of time.
To let him freely get away would make my skin crawl for months. All I would do is wonder about his next victim and know that I could possibly have prevented it. Sometimes, you just have to take a stand.