obvs i can't back this up with anything but speculation from first principles, but:
among the space of possible mental states, "having difficulty imagining particular mental states" is a prime candidate for the kind of mental state involved in this tragedy❗
if he was guilty: his inability to imagine that that those he was hurting were actual humans
if he was innocent: his inability to imagine that those who set him up were actual enemies
"unthinkable" is a scam, reject it