Idiom similar to wolf in sheeps clothing?

Solution 1:

The idiom "snake in the grass" may work well for you.

This idiom continues the theme of animals hiding and waiting to attack. However, snakes are terribly misunderstood and virtually none intend to harm humans (in fact numerically few are even capable of harming a creature as large as a human.) Despite this, snakes are vilified as incredibly destructive; so much so that the idiom in question implies that if a person were to walk in tall grass where a snake was located, then something terrible would happen to that person.

Solution 2:

Been awhile since you posted your question and I don’t know if you ever finished your lyrics, but when I came across this random post I immediately thought of ‘you can’t make a silk purse from a sow’s ear’

The meaning that you can’t change people- make them into something they’re not, somehow resonated for me with the concept of those who pretend to be something they are not. Good luck!!