What makes a person act against injustice?

Everyone interprets the word justice differently, but personally I think justice integrates moral values and high moral conducts according to the values of the society. On the other hand, injustice means violating the laws and standards set by the society today such as crime. Therefore, people who act against injustice are heroic and courageous. However, there are distinctive reasons of why some people would act against injustice while others do not, being bystanders themselves. Firstly, one's occupation matters; a policeman is obligated to "catch" and "handcuff" criminals who have committed serious crimes. Secondly, one may act against injustice if he or she is "selfless enough", and feels the need to resist injustice; such act is heroic. Thirdly, people seem to be "following the crowd" a lot. If one sets an example of acting against injustice and others showed their support, many would act against injustice too. Furthermore, a society's religious values is crucial in defining "injustice". There are certain acts that are perhaps condemned and banned by the religion a country believes in; therefore many would act against injustice in order to protect their religion and be faithful and pious.
When my brother and I were little, we used to fight a lot. When my brother was two, I was five. One day we argued over some toy while we were on our back home. I got really mad and sort of pushed my brother, and he fell on the marble stairs and knocked his head on the wall. He started to bleed and then my mom took him to the hospital for a small surgery (just for the cut to heal ) , and I never confronted my mom about the actual story, although my brother fell on the ground himself I had responsibility too. I still feel guilty !