Getting dabbed on as you bleed out

Or maybe he was just angry and scared enough that the chemical cocktail in his body provoked unpredictable behaviors that shouldn't be held against him considering he had the presence of mind to not inflict any unnecessary damage.

Could be either or both. But also worth considering that just because a person can wield a gun doesn’t mean they fully have the ability to process the situation. Kids are still developing. So shooting someone doesn’t necessarily have the same weight it might to an adult, who has finished developing. Also, developmentally speaking, teenagers are at an all time low for empathy (not kids these days, just people in that developmental age range). So, it could just as easily be that he really isn’t able to emphasize with the guy he shot.