Lord Krishna,being the impartial Supreme Lord, took a vow to not lift any weapons in the battle of Kurukshetra against any party. Still He lifted a chariot wheel to kill Bhishma and even took up His Sudarshan Chakra to protect the embryo in the womb of Uttara(Abhimanyu’s wife) from the Brahmastra released by Ashvathama to destroy even the last remaining descendent of the Pandavas.
On the other hand,we see a person like Karna,who when favoured by Duryodhna, never left his company and assisted him in every possible way. Even though Lord Krishna tried to convince him before the battle to give up Duryodhan’scompany,he would not budge even an inch from his vow to support Duryodhna.
How do we understand this? The Supreme Law maker breaking his vow and a mere human being refusing to do so even when the Lord personally trying to convince him.
The Supreme Lord Krishna,being the supreme authority,gives the religious principles and whatever He says becomes Dharma and anything against His teachings constitutes Adharma or Irreligion. The Lord,sometimes, breaks His vows for a higher purpose. Karna had some understanding of Dharma but “NOT DEEP ENOUGH”. What he did not know was that “THE OBLIGATION IS ONLY BINDING WHEN THE RESULT IS GOOD”. Duryodhan fought the war of irreligion depending solely on Karna. And where did it end? 64 Crore people Killed..! Wholesale massacre…!If Karna had listened to Krsna,the entire slaughter could have been avoided.ButKarna chose to follow the propagator of irreligion,Duryodhan, rather than the upholder of the religion,Lord Krishna.
Lord Krishna,on the other hand,gave up His own vows when He saw that attachement to His vows could lead to disastrous effects in the form of the symbols of irreligion like Duryodhana and his followers,becoming prominent. He preferred to be called as Bhakta-Vatsala(the one who is very affectionate to His devotees) rather than a worldy moralist who never breaks his solemn vow,not understanding that “THE OBLIGATION IS ONLY BINDING WHEN THE RESULT IS GOOD”.