Vaasudev told this story to Arjun in reference to "When Arjun could not kill Karn, Yudhishthir got very angry and fie him saying ":What is the use of your Gaandeev? If you cannot use it, give it to some other king who is superior to you...." In fact Arjun had taken a secret vow that whoever will tell him to throw his Gaandeev, he will kill him; so here, as Yudhishhir said thus, he got ready to kill him. Then Krishn pacified him with the following two stories -

Story of Balaak Hunter

[Ch 69] There lived a hunter whose name was Balaak. Although he didn't want to do it, but he had to kill animals to feed his family. He supported his parents and some other people als who depended him. He always spoke truth and did his own duties. One day even after a long effort he could not find any prey. At last he saw a beast who was blind. Although he had never seen such an animal before, still he killed him. As he killed him flowers showered on the hunter from the skies. A Divine vehicle along with Apsaraa's singing came there to take that hunter.

That beast did intensive Tap and got a Var so he used to kill many creatures that is why by Brahmaa Jee made him blind. After killing that animal Balaak had ended the killing of other animals. Thus Balaak went to Heaven. In fact morality is very difficult to understand.

Story of Kaushik Saadhu

There was a Saadhu named Kaushik. He didn't have much knowledge of scriptures. He lived far from village and there met several rivers. He made a vow that he would always speak truth. So as a speaker of truth he became very famous. Now people started hiding in that woods from the fear of robbers. And robbers also came there in search of them, They used to ask Kaushik to tell them about those men in the name of the truth, and Kaushik had to tell them about those hidden people. Those robbers caught them and killed them all. In this story Kaushik was a sinner. He was ignorant of the subtleties of the morality, he fell into Hell. Even as a foolish man, a man of little knowledge, he fell into Hell because he did not ask the solutions of his doubts from aged people.

Morality has been developed by learned people for the growth of creatures. There must be some indication to distinguish the virtue from sin. Sometimes it is attained by practice of morality, some say it is written in scriptures. I do not contradict it, but Scriptures, however, do not provide every case. Dharm protects and preserves the people so it is the decision of Pandit as what maintains the Dharm.