Crime certainly doesn't lose it's motive; to take what you want. Defying the law isn't the purpose hardened criminals have behind breaking said laws. In fact, crime has bigger motivation when there's no law to stop you other than the people you're doin' wrong, and the necessities for survival are scarce.
My point is, guns and numbers are power. The most common person to have either and especially both are criminals and gangs. Psychopaths and villainous scum aren't gonna have second thoughts about ravaging the supplies of survivors and markets and such during the first days of the apocalypse.