bad egg -------- 坏蛋（俚语）

英文释义

(SLANG) Someone or something that disappoints; a bad person.

例句
We liked the friendly new accountant when we hired him, but when we discovered he was stealing from us, we realized what a bad egg he was.
在招聘这位和善的新会计师时，我们都喜欢他，但是当发现他正窃取我们的钱财时，才意识到他是怎样的一个坏蛋。