Although Arjuna knew that Kṛṣṇa was his friend and the Supreme Personality of Godhead, he was nonetheless puzzled by the various forms exhibited by Kṛṣṇa. Therefore he asked further about the actual mission of this devastating force. It is written in the Vedas that the Supreme Truth destroys everything, even Brahmā. Yasya brahme ca kṣatram ca ubhe bhavata odanaḥ/mṛtyur yasyopasecanaṁ ka itthā veda yatra saḥ. Eventually all the brāhmaṇas, kṣatriyas and everyone else are devoured by the Supreme. This form of the Supreme Lord is an all-devouring giant, and here Kṛṣṇa presents Himself in that form of all-devouring time. Except for a few Pāṇḍavas, everyone who was present in that battlefield would be devoured by Him.
Arjuna was not in favor of the fight, and he thought it was better not to fight; then there would be no frustration. In reply, the Lord is saying that even if he did not fight, every one of them would be destroyed, for that is His plan. If he stopped fighting, they would die in another way. Death cannot be checked, even if he did not fight. In fact, they were already dead. Time is destruction, and all manifestations are to be vanquished by the desire of the Supreme Lord. That is the law of nature.