Sadism.
Did he pass his required psych eval before he was hired?
Did he fail the psych eval, and was hired anyway?
If so, who hired him? Who gave a known sociopath a gun, a badge, and life or death authority over everyone he meets?
Hold them accountable in a court of law for reckless endangerment resulting in a loss of life.
Giving a psychopath a gun and power is no more reasonable and prudent than throwing an enraged rattlesnake into a kindergarten.