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”.