mean streak
An inclination, tendency, or propensity for cruelty, spite, malice, or violence. Her dad is usually a pretty nice, chilled out guy, but he gets a real mean streak when he's been drinking. I think you should take your son to a therapist—he's got a mean streak that is very uncommon in a boy his age.
Farlex Dictionary of Idioms. © 2024 Farlex, Inc, all rights reserved.