I completely agree with you, I was just forwarding one mainstream theory. Obviously none of us know anything about this guy's personal psychology, but if you look through the eyes of psychologists, academics, practitioners etc., there is consensus that violent murderers are not psychologically well-formed.
This is from the DOJ in the US (
Psychopathology of Serial Murder: A Theory of Violence | Office of Justice Programs):
"The serial murderer is examined in the context of the "Diagnostic and Statistical Manual of Mental Disorders." The theory developed consists of three separate stages. Stage 1, called the "foundation of pathology," involves biological predisposition, environmental trauma and stressors, the development of esteem and control problems, and sexual dysfunction. Stage 2, called the "path of stressors and development to first murder," focuses on the development of maladaptive coping skills, retreat into a fantasy world, the commission of lesser crimes, the dissociative process, and finally the first kill. The third stage, called the "obsessive-compulsive and ritualistic cycle," involves the renewed urge to kill after the first killing, a cooling-off period, and the continuation of the cycle."
The so-called "dark triad" of personality disorders - Narcissist, Borderline, Antisocial - all share those three things I've highlighted in bold: 1. Esteem and control problems, 2. Maladaptive coping skills, 3. Dissociation. It's not a leap to diagnose someone displaying those traits with a PD, though of course PDs can be confused with things like bipolar disorder, substance abuse, and even biological issues.
The prison population is also an interesting place to look. According to NICE, in the UK an estimated 64% of male sentenced prisoners and 50% of female prisoners have a personality disorder (NICE,
https://www.nice.org.uk/guidance/ng66/documents/mental-health-of-people-in-prison-draft-scope2).
Academics in Iran found that 87.3% of women and 83.3% of men had a personality disorder at the time of committing the crime they were convicted for (PubMed,
The Relationship between Personality Disorders and the Type of Crime Committed and Substance Used among Prisoners).
I think the overriding point for me is that violent offenders are not behaving 'normally', psychologically-speaking, and there has to be reasons for that; either psychosis, malformed emotional development, acute substance effects, etc.