Since you seem to be looking for a phrase, I think this quote from Shakespeare would do nicely:
One may smile, and smile, and be a villain.
The phrase comes from Act 1, Scene 5 of Hamlet. In this scene, Hamlet discovers that Claudius, his seemingly-friendly uncle (and stepfather), murdered Hamlet's father to seize his throne (and Hamlet's mother). On learning this, he describes Claudius like so:
O villain, villain, smiling, damnèd villain!
My tables1—meet2 it is I set it down
That one may smile, and smile, and be a villain—
1 tables: writing tablets
2 meet: fitting, proper
You can see how the phrase fits your criteria: it describes a person who is nice and pleasant to your face, but is doing terrible things to you in secret (like murdering your father).
The main drawback is that the phrase is not common enough to be called an "expression" or an "idiom". If the listener doesn't catch the literary reference, the phrase loses some of its impact, since the words of the phrase don't actually say who the villain is hurting. Fortunately, most English speakers have studied Hamlet in school at some point, so there is at least a chance that they will recognize it.
One last note: Shakespeare's definition of "villain" was not quite the definition we use today. In his usage, it describes someone who lacks noble qualities, in both senses of the word "noble": that which is good and admirable, and that which is characteristic of the nobility. (These concepts were more synonymous in Shakespeare's time than they are today.) See this question from the English Language & Usage StackExchange for further details. Anyone who hears you say the phrase will most likely assume the modern definition of "villain" ("a deliberate scoundrel or criminal"), which is close enough.