It was Bhima's vow to kill all sons of Dhritarastra. We have seen Pandavas injuring them but not killing--leaving them to Bhima to kill. However in Karna parva it says:
He was then surrounded by ten heroic and foremost of warriors, viz., thy sons, all of whom were Duhshasana's juniors in age. ... Then with some broad-headed arrows he cut off and felled their heads decked with lips bit and eyes blood-red in rage.
Does it mean Arjuna killed 10 and not Bhima?