hanisyami balam hy etad
bhuvi bharam samahitam
magadhas tu na hantavyo
bhuyah karta balodyamam
[The Supreme Lord thought:] Since it is such a burden on the earth, I will destroy Jarasandha’s army, consisting of aksauhinis of foot soldiers, horses, chariots and elephants, which the King of Magadha has assembled from all subservient kings and brought together here. But Jarasandha himself should not be killed, since in the future he will certainly assemble another army.
After due consideration, Lord Krsna decided that since He had descended to the earth to destroy the demons, and since Jarasandha was so enthusiastic to bring all the demons to the Lord’s front door, it was definitely more efficient to keep Jarasandha alive and busy.