笑里藏刀 (xiào lǐ cáng dāo): To be friendly on the surface but treacherous underneath — Meaning & Stroke Order

Don't be fooled by his enthusiastic appearance; he's a person who hides a knife behind a smile.

In business, be wary of competitors who hide a knife behind a smile.

This story tells us to beware of petty people who hide a knife behind a smile.

During the negotiation, the opposing representative hid a knife behind his smile; the conditions he proposed seemed favorable but were actually traps.