mean streak, a

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's 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.
See also: mean, streak

mean streak

Fig. a tendency for a person to do things that are mean. I think that Wally has a mean streak that no one ever saw before this incident.
See also: mean, streak

mean streak, a

A tendency toward nastiness. In effect, the phrase implies that a person is not entirely bad but is given to occasional spiteful or malicious behavior. The Toronto Daily Star used it on June 16, 1960: “A girl who would be attracted to Bud’s mean streak and bad temper must be a little out to lunch.” See also out to lunch.
See also: mean