Personally, I think that people who commit mass murder do so as a form of stress relief. The kind of stress they have is usually emotional, not physical. Another factor could be their inability to accept that they are at fault for whatever is wrong in their life.

Take for example some of the recent mass shooters. A few of them had romantic problems. Some of them couldn't even have sex with people because they're that unlikeable. These people are now popularly called "involuntarily celibate" or "Incel". Incels are usually men who fail to get intimate with women. Sometimes they're simply unattractive. Sometimes they have extreme views on how women should behave, which women don't like. The incel thinks that he's a "good guy" and he "deserves" to be loved, and all women are suddenly bad or they're whores. He blames everyone and everything except himself for his failure to find love or even make friends, or hold a steady job. He starts truly believing that the world is at fault and he is the only one who is right, and so in an act of vengeance, he goes out to kill other people. He believes that other people are always successful and he's the only one who isn't, so this kind of pointless jealousy of other people's success in life is what eventually makes the incel kill random people.

I think deep down they know they're the ones really at fault, but they just don't have the guts to own up to the fact and they're unwilling to accept the truth or change for the better.