Even a worm will turn

Metaphorically  worm is an insignificant or contemptible person. Meek and humble as he is, however, he will resist or retaliate if pushed too far.

‘You wouldn’t think little Percy Perkins had it in him, but on the bus this morning that cantankerous wife of his tried to pick a quarrel with the conductor, and Percy told her in a loud voice to shut up.’

‘Good for Percy! Even a worm will turn.’