A kid who was jilted texts his friends that he has bought an assault weapon, repeatedly refers to himself as a killer, and claims to have a plan to commit mass murder at a huge party, where he will "kill the bitches",and you call that a "potential warning sign"?  No, it was an overt threat, a declaration of intent, specific in its goals.  As such, it was an obvious and serious threat that should not have been ignored.  But it was ignored, and three people at the threshold of adult life are dead, and another shot up with bullets, an extremely painful condition.  A bunch of people let them down, and so they are dead.

And here you are, apparently insinuating that the girlfriend "did (something) to him".  Sounds just like blame the victim.  Rejection by a mate raises strong emotions that too often result in suicide or homicide.  We all know that.  Couples break up all the time, but that shouldn't result in death.  When friends see that situation develop, they have to intercede.