At least he didn't attack me, and he even lent me his cell phone, he was such a gentle criminal.

criminal 是名词，由 crime（犯罪）加后缀 -al 构成，表示"犯罪的人"或"罪犯"。后缀 -al 常用来将名词或动词转化为形容词或名词，这里是将名词 crime 转化为指人的名词。