This chapter aims to provide a review of the literature on the role of personality traits and disorders among terrorist offenders, as well as extant terrorism risk and threat assessment (TR/TA) instruments. We assert that there is an overwhelming need for an instrument that is largely based on DSM-5 personality disorder dimensions and related traits. Specifically, an assessment tool is proposed based largely on the Personality Inventory for DSM-5 (PID-5), in combination with domains borrowed from the Comprehensive Assessment of Psychopathic Personality (CAPP), as well as accounting for ideology and prior criminality. Using open sources, we discuss the prevalence of the included traits in both Omar Mateen and Dylann Roof and argue that their unstable personalities could have led investigators to downplay the risk they posed given that they break the mold of the terrorist as having a stable personality who methodically seeks to avoid detection.