The Almighty does all things for a reason, which support Dharma. Be it pleasing his devotees, or punishing the wrong doers, all actions of his have a cause which makes one understand the importance of Dharma even in its minute detail. Such reasons were many in the Kurukhestra, wherein Shri Krishna’s actions were surprising, but in the end, all were to support Dharma which is above everything.
The Kurukshetra War is the epic war of Dharma against Adharma. Shri Krishna himself was on the side of Dharma supporting the Pandavas. But, he was not on the fighting verge and took a vow that he would not yield a weapon. Before the war starts, the Pandavas come to the camp of Bhishma Pitamah to seek his blessings in the war along with Shri Krishna. Being an ardent devotee of Shri Krishna, he understands what the actions of the Almighty are, yet challenges that he would make Shri Krishna yield a weapon in the war for once. Shri Krishna smiles, but says nothing.
The war starts and both the sides fight relentlessly for victory. Although fighting for Dharma, it was not an easy task for the Pandavas as they had to fight great warriors like Bhishma Pitamah, Dronacharya, Kripacharya and others to win. The wrath of Bhishma Pitamah was especially very terrifying and none of the Pandava warriors nor army could face his terrible arrows and fight him. This went on for few days, and Shri Krishna knew that unless Bhishma Pitamah succumbed, the Pandavas hardly had a chance to win.
The next day, Arjun faces Bhishma, but is unable to take the force of Bhishma’s arrows. He fights hard to counter Bhishm’s arrows that were hurting not only Arjun, but Shri Krishna and the horses too. Enraged at Bhishma’s war and in a fit of anger, raises a chariot wheel and moves forward to Kill Bhishma. Seeing the Almighty. Bhishma leaves his bows and arrows and bends in front of Shri Krishna and says that it is indeed a blessed privilege that he be killed by the Almighty himself. But Arjun rushes forward and stops Shri Krishna from attacking Bhishma, pleading and reminding the Supreme Being not to break his vow. It is when Arjun promises that he would defeat Bhishma Pitamah that Shri Krishna goes and mounts the chariot again.
The reason for Shri Krishna getting ready to break up his vow was not for the hatred on Bhishma, but for the affection on him. Bhishma was his ardent devotee, and when his devotee took a pledge that he would make Shri Krishna yield a weapon in the war, it became the duty of Shri Krishna to fulfil the wish of his dear devotee. Thus, Shri Krishna caused his actions to make him break his vow and yield a weapon. However, with the request of his yet another dearest devotee Arjun, he again gives up the weapon and takes his position back as the charioteer of Arjun. This act only shows the earnest of Shri Krishna to uphold Dharma in all ways, one by getting ready to fight the warrior Bhishma, and the other fulfilling the wish of his devotee Bhishma.