Dang, Champ, that is awful. Was it an employee, or former employee?
I'm pretty sure Las Vegas has a significant rate of solo suicide attempts and completions. During the Hoover Dam tour years ago they provided that very grim statistic, as applies to "jumpers" at Hoover Dam and suicides elsewhere in the city.
A different profile from the person who completes mass murder/suicide.
Forensic examiners of mass murder-suicides paint a very grim prospect of being able to identify and prevent. Diaries and missives left behind reflect traits of hostility, grandiosity, feeling entitled, a strong sense of victimhood and strong drive to punish who they see as "tormentors." Conversely, they want their murderous act to be viewed as grand and glorious.
Often they have been seen by medical and mental health professionals, and often have filled out requisite personality inventories that, in looking back, MIGHT have flagged the traits of this severity of instability.
But our cultural of respect for civil liberties, not to mention privacy laws insure that, for the most part, the mass murder/suicide candidate will seldom be profiled and detected before he makes the grand gesture. Apparently there were red flags on that German pilot's mental stability Just not recognized or acted upon.
The fact of the M Casino guy leaving the detailed manifesto, naming specific "tormentors", that he had met with a news reporter more than once wanting his complaints against M aired publicly, had been arrested for stalking one of the M employees, hospitalized in a psych unit (was he court committed?)...talk about your red flags.
But what would anybody do, any one person or organization, if they had all these flags put together on the M guy? Which is unlikely anyway. He was already fired, from M, was he not? Are fired employees assigned a no trespass status? Would that prevent one from walking in with a gun and killing patrons before they were stopped?
M casino certainly dodged a bullet, to use a very bad pun. I suspect that a rather large percentage of people who are fired from a job either already have, or develop hostile, if not murderous, feelings toward the workplace and certain people. Probably what contributed to their being fired.
What to do about it?