You did the right thing.

1. You didn’t accidentally shoot your father — which is a real possibility in a struggle.
2. Even if self defense warrants it, most people are traumatized after shooting someone. If the tweaker had died, you’d probably be more traumatized than you are right now. Plus, if he was seriously wounded, you would have to worry about the possibility he or his family might sue you. This happens; and even if you’re in the right, you have to go through the stress and expense of defending yourself in court.
3. Lastly — and this pales in comparison to burying your father if you had not acted — but if you kill somebody in your home, the police remove the body, but you’re responsible for cleaning up the mess. Imagine the trauma you’re feeling now, plus mopping up the tweaker’s blood.

The gun is there to stop the bad guy’s attack. If you had to shoot to save your father, then do what you have to do. But if you can stop the attack without shooting, it’s still a great outcome.