even a worm will turn

Etymology
First attested in a 1546 collection of proverbs by, in the form “Treade a worme on the tayle, and it must turne agayne.” Popularized by in The Third Part of Henry the Sixth, see quotations.

Proverb

 * 1) Even the meekest or most docile people will retaliate or seek revenge if pushed too far.