Is it a mental illness? Are they brain damaged in some way? Perhaps they're an older form of human set loose in this time in error ~ Ergaster ~ one of them died they just left 'em lying there and stepped over the body to continue hunting and gathering.
“Is it a mental illness? Are they brain damaged in some way?”
No, it’s just evil. It is not fashionable to call evil what it is, these days. We want to find some other reason like illness or damage. But it’s evil, and that is scary.