The ruthless enemy killed the old lady.残忍的敌人杀害了那位老太太。 His business rivals knew they could expect no quarter from such a ruthless adversary.他的商业对手都知道不可指望他这样无情的人会手软。 It's obvious from his ruthless behavior that he has no morality.从他的行为可以看出他毫无道德。
用作表语
▲ S+be+～+to-v
We had to be ruthless to make this decision.我们不得不坚决地作出决定。