How to describe your feelings when someone else is treated unfairly?

Solution 1:

I would likely describe your sympathetic response as, "righteous indignation."

RIGHTEOUS adjective:

1. (of a person or conduct) morally right or justifiable; virtuous. "he is a good, righteous man, I am sure"

synonyms: good, virtuous, upright, upstanding, decent; see Google righteous

INDIGNATION noun: anger or annoyance provoked by what is perceived as unfair treatment. "the letter filled Lucy with indignation"

synonyms: resentment, umbrage, affront, disgruntlement, displeasure, anger, outrage, annoyance, irritation, exasperation, vexation, offense, pique. see Google indignation