Strong, formal/literary register - typically describes rulers, regimes, or cruel acts (暴虐统治 'tyrannical rule', 暴虐行为 'brutal acts'). For everyday cruelty in casual speech, prefer 残忍 (cánrěn, 'cruel') or 凶狠 (xiōnghěn, 'fierce').
memory
暴 ('violent / sudden') + 虐 ('to mistreat / abuse') = 'violent abuse' = brutal, tyrannical. The same 虐 appears in 虐待 ('to abuse/maltreat') and 凌虐 ('to bully / tyrannize'). The character 虐 has the 虍 (tiger) radical at the top - fittingly fierce.
