That guy who betrayed his friends is truly a black-hearted person without any remorse.

black-hearted 是由形容词 black（黑色的）和名词 heart（心）加上后缀 -ed 组成的复合形容词。black 在这里象征"邪恶、阴暗"，heart 指"内心、本性"。black-hearted 字面意思是"内心黑暗的"，引申为"心肠歹毒的、黑心的"。