Krishna is the lord of the all creations and His actions are never fully understood by mere mortals. However, there can be many meanings extracted from His actions and teachings, and one may choose the ones according to his/ her taste.
Krishna took a vow that He will not take up a weapon in the famous war of Kurukshetra. At the same time, His great devotee, Bheemsen (one of the eight Basus) vowed to give a fight to the Pandavas that Krishna will be compelled to break His promise.
While fighting, Arjuna was not able to match Bhishma s skills and strength. In that particular time Krishna appeared to attack Bhishma with a chariot wheel, breaking His own pledge.
Krishna, being Achyuta, can never break a promise. But when conflict occurred with His own vow and His great devotee, Bhisma s He apparently broke His own to keep the devotee s. That is the sweetness of the Lord. He loves all of us more than He loves Himself or we ourselves.
Actually Krishna did not break His own promise too, as a chariot wheel is not a weapon in true sense. So, He maintained a very fine balance so that both the vows taken up by Him and His devotee are honored.
I ask for forgiveness in advance as if I ve written anything incorrect with my small knowledge and smaller understandings.
Jai Shri. Krishna.