Глядит овцой, а пахнет волком

The man only looks friendly or harmless, but he is really an ill-doer. See В тихом омуте черти водятся (B)

Cf: The cross on the breast, and the devil in the heart (Br.). The devil lurks (sits) behind the cross (Br.). Outwardly a lamb, inwardly a wolf (Am.). Vice is often clothed in virtue's habit (Br.). Vice knows she's ugly, so she puts on her mask (Am.)