I’m writing lyrics for a song about xenophobia and have a line that I like “Just because there has been one wolf in wool, doesn’t mean we all have fangs” I’d like to follow that up with another deconstructed idiom like that but I hit a wall. Any ideas? Thanks!
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.
There is the common metaphor of rotten apples. The most common version is, "One rotten apple spoils the whole bunch." But in relation to your context, the Osmonds challenged that notion in a lyric:
One bad apple don't spoil the whole bunch, girl
Give it one more try before you give up on love
It does not, however, include the element of deception that "wolf in sheep's clothing" does.