In the childhood, he killed the tyrant king Kamsa, who was his uncle, as he was a great adharmi. Before that he killed many rakshasas sent by Kamsa to kill him because they also were all adharmis, just like Kamsa.
After killing Kamsa, he installed Kamsa’s father Ugrasen as the king of Mathura again instead of becoming king himself.
When Jarasandh attacked Mathura to take revenge on him for killing Kamsa, he killed his entire army but spared him so that he could again collect more adharmis and come again to kill him.
When he saw that people of Mathura could not live peacefully due to repeated attacks of Jarasnadh, he decided to leave Mathura and established the new city of Dwarka in between the sea so that none could attack it easily.
When he fled from Mathura in front of Jarasandh, Jarasndh insulted him and called him ‘ranchhod’ but Lord Krishna did not care for his insult as for him the welfare of people was much more important.
He married all 16,100 girls who were kept in captivity by Narkasur so that they can lead a respectful life in the society as nobody else was ready to accept them.
He helped Pandavas whenever they were in distress as they were righteous people.
He went to the Hastinapur royal court as the peace messenger on behalf of Pandvas as a last attempt to avert the war where he was insulted by Duryodhan and also tried to capture him.
He helped preserve the modesty of Draupadi when she was being disrobed in the Hastinapur royal court.
He was strictly against taking of any pledge by any person as the pledge may prove to be harmful in future.
In case of Bhishma, his pledge of leaving the throne of Hastinapur for his step mother Satyavati’s future sons proved to be very harmful for Hastinapur.
Drona also gave more importance to Dhritrashtra’s help and supported him despite knowing that Duryodhan was on the wrong path. Same was the case with Karna who gave more importance to Duryodhan’s friendship than dharma.
He agreed to become the charioteer for his best friend Arjun and guided & mentored him by reciting the divine knowledge of Geeta when Arjun was in a great dilemma about his duty.
He rebuked Arjun on the 3rd and 9th day of war for fighting very mildly with Bhishma and himself picked weapons to kill Bhishma as it was important for victory of dharma.
He helped Arjun reach Jaydrath and told Arjun about the boon of Jaydrath’s father to him and guided and motivated him to kill him before sunset.
He saved Arjun many times during the war, helped Pandavas in eliminating Bhishma, Drona and Karna.
He motivated Yudhishthir to kill Shalya and signaled Bheem to kill Duryodhan by striking his thigh to finally end the war.