The fulfillment of one's promise is one's duty. Formerly Bhima had promised in the midst of the assembly that he would in great battle break with his mace the thighs of Duryodhana.
सुयोधनस्य गदया भक्ताम्यूरू महाहवे। इति पूर्वं प्रतिज्ञातं भीमेन हि सभातले॥