During the fierce fighting, Balarama was about to kili Jarasandha, when a heavenly voice was heard that Balarama was not the person to kili Jarasandha. Therefore Jarasandha was set free by Balarama and Sri Krishna. The enmity between Sri Krishna and Jarasandha was lifelong. They confronted each other eighteen times. Ultimately Sri Krishna decided to kili Jarasandha in a duel. Once Sri Krishna, along with Arjuna and Bhima went to the city of Magadha in the guise of Brahmins.
They were received with traditional courtesy by Jarasandha, who wanted to know about their identity. Sri Krishna told him that his friends were having the vow of silence, which would terminate only at midnight. At midnight, Jarasandha came again to know the identity of the three Brahmins. Then Sri Krishna revealed to him that they were Krishna, Arjuna and Bhima and had come to fight a duel with him. They wanted to set free many innocent kings, who had unreasonably been put in a dungeon.
Jarasandha was asked to select any one of them for a duel. He, in a great fury, selected Bhima, who killed him in that duel. The tyrant had to be punished and for this purpose, the method of duel-fighting was chosen, so that the unnecessary warfare could be avoided.
1. Kohli, Surindar Singh (ed), Dictionary of Mythological References in Guru Granth Sahib, 1993