Krishna had killed Barbarika, the grandson of Bhima, after watching his archery skills. Barbarika could have ended the war within a single day.
Although Barbarika had vowed to take part in the weaker side, which mean than initially he would have fought with Pandavas. Then, with the Kauravas and then with the Pandavs again.
But, Krishna could have asked Barbarika to forego his vow, and fight for the Pandavas.
