You tend to have another view about scum that would break into your house or do you harm anywhere after you've been the victim of it before.
And you seem to have another view about scum that would drag your already shot, incapacitated, and gasping for air body across the room, put a pistol under your chin, and fire a round into your brain.....
That is a really fucking huge (like Mount Everest fucking huge) difference between accidentally or even purposely putting an extra round in the badguy
during the altercation "for good measure".
The altercation was completely over and by his own admission he mindfully drug her body next to her companion while she was gasping for air, stuck a gun under her chin as she lay on the ground, and pulled the trigger. That is fucking murder anyway you try to slice it. If she was even remotely a threat he would have shot her BEFORE dragging her, dontcha think?