I'm not a psychologist so maybe the word "psychosis is not the right one
What I'm trying to say is that if you kill somebody you don't know, somebody who did nothing bad to you or your family, somebody who is a perfect stranger to you (i.e. if you're a mass murderer or a serial killer) you're not normal. Your brain is not normal.
I don't think they're losers. 99% of the people in this world live a miserable life (or almost miserable), you can call them "losers" but they don't kill others.
I don't even think they're sexually frustrated because in these times sexual freedom is almost unlimited. Pretty much everybody can make sex with somebody. Even if nobody likes you, you can always find out somebody (via prostitution) or use your "imagination" (via *advertiser censored*).
I think these shooters are really...what can I say... "subnormal" and nothing else