So I watched the Slenderman documentary and my initial suppositions were that a demonic presence was what contributed to the stabbings. Things get complicated though when it turns out one of the girls suffered from schizophrenia, which contributed to her delusion that she had to stab her friend to death to appease the Slenderman.
This creates sort of a moral conundrum.
Can an evil act happen while the actor is simultaneously not evil or less accountable in the evil act? Where is God's judgment in dealing with the biologically impaired?