You're wrong. According to Minnesota law:
609.065 JUSTIFIABLE TAKING OF LIFE.
The intentional taking of the life of another is not authorized by section 609.06, except when necessary in resisting or preventing an offense which the actor reasonably believes exposes the actor or another to great bodily harm or death, or preventing the commission of a felony in the actor's place of abode.
You are only authorized to use deadly force if you reasonably believe you are "expose[d] . . . to great bodily harm or death," or that you are "preventing the commission of a felony" in your home. In this case, that (a) clearly applies to the first shot(s) taken at the male intruder and then at the female intruder, (b) may apply to the subsequent shots taken at the male intruder, and (c) clearly does NOT apply to the subsequent shots taken at the female intruder. The shooter clearly admitted as such to police:
Smith stated several minutes later he again heard footseps on the main floor of his house. Smith stated that another person came down the stairs into the basement where he was sitting. Smith said he waited until he could see her hips and then he shot her. After shooting the person she stumbled down the steps. Smith stated that he tried to shoot her again with hte Mini 14 but the gun jammed. Smith stated that, after the gun jammed, the woman laughed at him. Smith stated that it was not a very long laugh because she was already hurting. Smith acknowledged that this made him upset. Smith told Investigator Luberts, "If you're trying to shoot somebody and they laugh at you, you go again." Smith then pulled out his .22 caliber 9-shot revolver that he was wearing, and he shot the female several times in the chest. Smith acknowledged that he fired "more shots than I needed to." Smith stated that he then dragged the female's body into his office workshop and placed her body next to the man's. Smth stated that the female was still gasping for air. Smith stated at this point he placed the handgun under the woman's chin and shot her "under the chin up into the cranium." Smith described it as "a good clean finishing shot."
Smith admitted that, after the female stumbled down the steps, he tried to shoot her again, his gun jammed, she laughed, and he got upset and shot her again with another gun. Then, he dragged her into another room, and THEN, when she was "gasping for air," he "placed the handgun under the woman's chin and shot her under the chin up into the cranium." At that point, there was no way he could have reasonably believed that his life was in danger or that he was preventing the commission of a felony. Castle doctrine or not, there is no self-defense justification for a "clean finishing shot."
His statement, particularly with regard to the shots he took at the female intruder AFTER she was gravely wounded, is nothing less than a confession to murder under Minnesota law.
Here in the state of Washington the presence of an intruder in your house is reason enough to shoot them to stop the threat. There are also cases of intruders fleeing and shot in the back (dead) as they are fleeing and the homeowner is not guilty. (Has to do with preventing the intruder from doing additional harm).
However, even in Washington State, treating them like a wounded animal with a “coup de grace” shot would be illegal.