He eliminated his enemies by brutal means.
残酷 is a more common word for 'cruel' and can be used in everyday contexts, while 残忍非道 is stronger and more formal, emphasizing both cruelty and lack of humanity.
非道 alone means 'inhuman' or 'wicked', but 残忍非道 combines it with 残忍 (brutal) for a more intense expression.
A compound of 残忍 (ざんにん, 'brutal, cruel') and 非道 (ひどう, 'inhuman, unjust'). The combination intensifies the meaning to 'cruel and inhuman'.