The vast majority of mass killings, no matter the claim, are suicide based and most often end up that way even if it takes many years.
Think about the airline pilot who deliberately crashes his plane into a mountainside. Why is that? I mean if he wants to commit suicide why not just shoot himself or jump off a bridge. Why take hundreds of innocent souls with him? The simple answer is that misery loves company.
The reasoning goes like this: Our ego always wants to blames others for our failings, though many mature folks manage to rise above ego and see things as they really are. But the latter is definitely not the case for the mass murderer.
When a potential mass murderer gets to the point whereby he (and it’s mostly always a he) sees his life as an irredeemably spoiled and wishes to die, he does not want to see it as his fault. The fact that he has made a lifetime of bad decisions or simply lacks the skills to succeed to the level that he thinks he deserves does not enter his consciousness. Rather it is because the world — people — has been against him. It is they who have ruined his life. Therefore when he feels that the time has come when he must die, he sees it as only fair that at least a chuck of the wicked world should die along with him.
The greatest mass murderer in history was Adolf Hitler. All his adult life Hitler had been on the edge of suicide. In fact he had made numerous attempts at ending his life, and ironically during one attempt it was a Jewish couple that blocked him. In the end Hitler caused 60 million humans to die before finally blowing out his own brains.
When mass murders embark upon their dastardly deed they either plan in the end of their mission to kill themselves or act out in ways that will force authorities to kill them. Either way it is primarily a suicide mission.
Though this scenario has been acted out countless times over the centuries, the most bizarre aspect of it all is humanity’s dumfounded reaction. Why did he do it? What was his motive? The answer is nearly always the same. He hated himself and the world that he believed ruined him. Anything else is mostly superficial and irrelevant.