Heartless
Part of speech: adjective
Definition: Without a heart; specifically, without feeling, emotion, or concern for others; uncaring.
Example sentence: The problem with certainty is that sometimes it can sound cold and heartless, although it is the most compassionate and supportive answer.
Obdurate
Part of speech: adjective
Definition: Stubbornly persistent, generally in wrongdoing; refusing to reform or repent.Physically hardened, toughened.