Seeing the injustice, he stepped in at once to stop it.
Tips
history
From 《水浒传》 Chapter 17: 俺却路见不平,直送他到沧州,救了他一命 - 'seeing wrong on the road, I escorted him all the way to Cangzhou and saved his life.' Almost always paired with 拔刀相助 ('draw sword to help') in the full form.
culture
Core value of 侠 - the wandering-knight ethic in Chinese martial-arts stories (武侠). Wuxia heroes from the Water Margin to Jin Yong's novels live by this code: step in when you see injustice, regardless of personal cost.