Psychopathy, the inability to feel guilt, is one statistically important predictor of mass murder, and psychopathy is significantly heritable, at an h(square) up to 0.74.  Two first-degree relatives of the Las Vegas massacre perp were highly likely psychopaths: his father, on the FBI Most Wanted List, and a brother.  That increases the probability of psychopathy in the perp.

Now we have the Daily Mail, which quotes an unnamed Las Vegas prostitute, who allegedly spent time entertaining Stephen Paddock, to the effect that he:

1. boasted about sadistic sex fantasies, or possibly sadistic acts. We are not told which one.
2. told her, "I was born bad."  (Self-reported psychopathy is an empirically supported sign of the diagnosis.)

This helps confirm the sheriff's use of the word "psychopath" to describe the murderer.  Sadistic delight can be a motive for mass murder, as in the works of the marquis de Sade.  Sadistic delight is marked in the Ekman-Friesen system as emotional smiling or laugher combined with an "evil frown" (contraction of eyebrow muscles).  The Batman series writers were probably aware of this, since Carl Jung long ago called attention to the "Joker" archetype in mythology.  This is a known motif in cultural psychopathology.

There are precedents in the mass murders of the 20th century, and Kim Jong-un is currently trying to display sadistic laughter in his N.K. videos.  I find Kim's facial expressions unconvincing, because they look voluntary rather than spontaneous.  He is willfully performing on those videos, and he is a lousy actor.  However, videos of his spontaneous facial expressions may exist.  He certainly has executed some of his victims with an anti-aircraft gun.  But that means he wants to be perceived as evil, which is not good evidence for genuine psychopathy.  Psychopathy can be faked.

However, the Las Vegas crime looks increasingly like a case of "sadistic" psychopathy, as indicated by the murderer's reported sexual fantasies.  Sadistic and masochistic sex fantasies are not uncommon; witness the huge sales of books like Fifty Shades of Grey to a largely female audience.  Mass murders are uncommon, but sadistic and masochistic sexual fantasies are not.

It is his decision to plan and work consistently over a long period of years to amass weapons, his very deliberate planning, etc., that would make this a prosecutable crime if he were alive.  This was mass murder with malice aforethought, and personality-disordered people are not psychotic.  They are rational – "just evil," as he said himself.

There is a traditional argument that the need for more extreme sexual stimulation escalates over time, very much like other addictions.  In more repressive cultures than ours, sexual escalation may be avoided or suppressed.  In our consciously and willfully sex-obsessed culture, where other varieties of traditional meaning and fulfillment are under constant attack, sado-masochistic and other "boundary-violating" types of sexual fantasies and practices are increasing in the public domain, and possibly in the private domain as well.  The Harvey Weinstein outing is just another example.  The dangerous rise in child sexual exploitation is another warning sign.

Other cultures have tried deliberate sexual indulgence, especially aristocracies (like the Romans beginning with Augustus), classical Greece with Alcibiades in "Symposium" (a discussion of sodomy with boys, much like Afghanistan), France at the time of the Revolution, the Weimar Republic, and post-WWII Europe, and now America.  The cultural decline is quite predictable, and the self-destruction of Western culture was predicted by observers like Nietzsche and Spengler.