A 16-40 year old underachieving male, using religion or other antisocial ideology to rationalize a nihilistic/anarchic desire to overturn the existing power structure (typical of low-status males throughout human history who's only means for increasing status was through violent revolution - eg jihadists and other brands of wanna-be aristocrats be they green, communist or fascist).
In polygamy practicing regions of the world (historically everywhere except western civilization) high status males monopolizing most of the women leads to unending cycles of beta male rage. When the society is militarily strong the rage can be directed outward through conquering new areas, raping their women, and taking them as wives, etc. However when the society is militarily weak beta male rage comes in the form of internal unrest, lack of social cohesion, civil wars, etc. The modern western world having removed the civilizing restraints on sexuality (marriage and monogamy) since the sexual revolution onward has seen social breakdown equal to polygamous societies with a major rise in mass casualties attacks by disgruntled single males. However unlike previous eras or other parts of the world internet pornography and video games has kept the worst at bay.
How come women, married men, or alpha males, never carry out mass shootings? Because it's beta male rage!