Most mass murderers do not go from zero to 60. Rodger made escalating assaults on women (splashing coffee on them, attempting to shove them off a ledge) before his killing spree. Both Cho and Justin-Jinich’s murderer harassed women before they killed anyone. When such acts go unnoticed and unpunished — because we expect men to harass women, and it’s not outrageous or even noteworthy when they do — they can become stepping-stones to more conspicuous and less socially acceptable acts of violence.
Interesting to note that while a history of animal cruelty is widely accepted to be a link with becoming a serial killer, the link between cruelty towards women and killing women is still up for debate. If a guy abuses a cat and then shoots women we say "we should have seen it coming that guy was nuts", but if abuses women and then shoots women we say "we had no way of seeing it coming that guy was a perfectly polite, kind and wonderful human”.
“THEN YOU SHOULD HAVE DIED!” roared Black. “DIED RATHER THAN BETRAY YOUR FRIENDS, AS WE WOULD HAVE DONE FOR YOU!”
“Actually,” said Harry, pocketing his e-cigarette, “Peter’s pursuit of rational self-interest is of a higher moral order than your determination to kill yourself on another person’s behalf, Sirius. Self-sacrifice is never the answer; it ends only in pain and death.”
Sirius blanched. “But Voldemort — we could have stopped Voldemort.”
“It’s a free market,” Harry said, shrugging.
Lupin turned into a wolf.
“Control yourself,” Harry said. “Good lord, man, you’re a being of pure will and drive. Exercise it.”
Lupin turned back into a man with flashing, clear eyes and a jaw that could level a mid-sized office building.
“In the marketplace of ideas,” Harry went on, “Voldemort has the same right to disseminate his philosophy as you do. If his philosophy is sound, it will flourish. If his philosophy is unsound, you have nothing to fear.”