Decent people have had their lives ruined by his behaviour.

decent 是形容词，由词根 dec-（来自拉丁语 decere，意为"合适、得体"）和形容词后缀 -ent 组成，本义是"得体的、正派的"，引申为"正直的、体面的"。在这里形容那些品行端正的人。