Someone left a comment in this thread (too lazy to look for) where they said people in some mass shootings are "spared for no apparent reason".
This is a really good but dark point.
When Randy went on his columbine larp in a grocery store, one of the employees who was a female was working or something with headphones in and wasn't realizing what happened just seconds ago, and he stared at her for a bit still holding his shotgun and decided to spare her.
It's unknown why but it happens sometimes. Like how during Columbine they had more than enough ammo to kill everyone in the library, but they chose not to. Who knows why, adrenaline? Maybe.
But in this case of course it was because the guy was white.