Nothing is good if it is not purposeful. That is the whole purpose of teaching Bhagavad-gita. Nothing is good; nothing is bad. Everything is good, everything is bad, in this material world, but we have to see. Just like the common phrase goes, "The end justifies the means." So that is Krsna teaching here that He has nothing to gain. He is full in Himself. But just to set examples in the world He was taking part in the fighting because He wanted to establish it that fighting for good cause should not be avoided. That was His mission.
Lecture on Bhagavad-gita 3.21-25, New York, 30 May, 1966