'Angry single male' fits well.
20-to-50 doesn't work that well when you consider the school-shootings, and I don't think middle-class covers the data completely either.
But angry, single males can be quite dangerous, whether they attack kindergartens in China, schools in Germany, Finland or the USA, random pedestrians in Japan or ex-wives and girlfriends in the US.
Angry, single females lacks the amount of testosterone that men have, so the relatively seldom times they do get violent, it usually only affects specific persons - or the womens children.