Insanity defenses are usually not successful, anyway. Depending on your state:
1. You have to show that you didn't know right from wrong and not be aware of the consequences of your actions - most states.
2. May have an added factor - your action can't be the product of mental
illness. Emotional impulse of the moment doesn't qualify as a mental illness. It might be a mitigating factor for a lesser sentence - maybe.
3. There are jurisdictions where you can be found mentally ill and guilty. Thus, you go to the hospital to get cured and then to prison.
My non-attorney reading of legal texts on self-defense indicates for our legal history and traditions, killing someone after they are not an active threat is legally and morally unacceptable.
PS - Idaho is eliminating the insanity defense and SCOTUS won't touch it.