A honey tongue, a heart of gall

दुर्जनैरुच्यमानानि
संमतानि प्रियाण्यपि ।
अकालकुसुमानीव
भयं सञ्जनयन्ति हि ॥

- हितोपदेशः

The words spoken by wicked people, though pleasant and agreeable, generate fear like the flowers blossoming out of season.

Appearances can be deceptive.
A honey tongue, a heart of gall.
Beware of wolves in sheep's clothing.