Devil in disguise noun phrase
Used to imply that someone seems to be good, useful, or benevolent but in reality they are not
Don't be fooled by his appearance. He is a devil in disguise.
Suddenly she is being helpful to me. I doubt that there is a devil in disguise.
John's just pretending to be nice to me. I know he is a devil in disguise.
The origin of this phrase is not clear.
If you swallow the bait, you take something that someone offers you, or agree to do something that someone asks you to do without knowing that it is a trick or way of getting something from you.
You can't just do whatever he wants you to do. Don't swallow the bait.