If Krishna was ready to break all rules and promises why didn't he just kill only Kauravas?
He was ready to fight against Bhishma and was ready to break promise by lifting a wheel. But why didn't he kill everyone at once? He even told Arjuna that he will kill everyone if Arjuna won't kill. Why didn't he just kill the Kauravas?
The Yadava clan (Bhoja, the Andhaka, and the Vrishni races) fought among themselves and died. Sri Krishna was witness to this incident.
Later, when Arjuna tried to bring the women of Vasudeva, robbers attacked him and he, being a great warrior, could not protect all the women.
Vyasa consoling him says as follows:
‘The mighty car-warriors of the Vrishni and the Andhaka races have all been consumed by the Brahmana’s curse. O chief of Kuru’s race, it behoveth thee not to grieve for their destruction. That which has happened had been ordained. It was the destiny of those high-souled warriors. Krishna suffered it to take place although he was fully competent to baffle it.
Govinda was able to alter the very course of the universe with all its mobile and immobile creatures. What need then be said of the curse of even high-souled Brahmanas? He who used to proceed in front of thy car, armed with discus and mace, through affection for thee, was the four-armed Vasudeva, that ancient rishi.
That high-souled one of expansive eyes, Krishna, having lightened the burden of the Earth and cast off his (human) body, has attained to his own high seat.
Sri Krishna did allow the war with a purpose - lightening the burden of the Earth.
This is the exact question which Arjun asked before the war. The fact is, Krishna could have done it, but if he did this all by himself, no one would have fought for Dharma. Everyone would've relied Bhagwan.
But if you talk of moral dilemmas of killing so many innocents, then let me tell you, many historical events took place to overthrow the sinful kings. And at that time, dying in a war is a blessing for a Kshatriya and the Kshatriya dharma. War does leads to destruction, it does destroys families and those connected to them. But it is their fate. It is a fate of every Kshatriya at that time.
This is the overall gist, Krishna told Arjuna, why he should fight. It is mentioned in Bhagwad Gita.
