Refers to a person who derives pleasure from inflicting pain or humiliation on others, often in a psychological or sexual context.
He is a sadist, so he enjoys bullying others.
サディスト is the noun for a person with sadistic tendencies, whereas サディスティック is the na-adjective describing sadistic behavior or traits.
Loanword from English 'sadist', ultimately derived from the Marquis de Sade.