If you have to fire upon a human the inevitable visit to a judge will occur. Now is the time to consider what you will say and how you will act. If you profess to be a "killer", the "other side" will glom onto that and paint you in a very, very bad light(Mr Zimmerman). If your "plan" is to "stop the threat" your intent is clearer. The first two rounds in my home defense shotgun are "rubber buck" for, as I will say to the judge, "Sir, I went out and hunted down a non-lethal round in order to "stop the threat" not "kill". The fact that RB rounds will shatter your chest at seven to ten feet is irrelevant, the intent is to be non-lethal. By "advertising" your intent to "kill" you increase the possibility of excessive jail time.
I was trained this way for many, many years and got quite good at it. Today, I cannot afford that much ammunition to train at that level. I no longer do the "head shot". Two, COM, will be a good start to stop a threat. Adding the HS may get an innocent injured and will be a hard target. With adrenalin pumping and stress increasing, that HS may be difficult and I do not recommend it to the untrained.