There is, in Christian literature once upon a time, a discussion wherein the child asked God why he needed to die, and God replied that He saved the child before he could sin and find condemnation, whereupon a great lament arose from the depths of Hell, and the cursed asked, "Why, Lord, did you not save us?"
The particular reference slips my mind; I forget its specific purpose, but it illustrates something, and the fragment I've recalled is incomplete.
Still, the idea that God would prefer self-destruction to the harming of the innocent ought not be so difficult, despite everything else, like the genocide that includes killing babies, or the bit with the angel warning Mary and Joseph off, so that their baby could escape the slaughter of the innocents; the Book of Matthew does not include any record of a rising lamentation from those not spared. Oh, right, and there was that slaughter of the firstborn, in Egypt.
It is also important to pay attention to everything else. Such as it is, the Disciples asked a silly question, and Jesus is just getting started with the millstone line. Stop fisking the Bible, and try reading the chapter as a contiguous rant; it's an interesting bridge between the Sermon on the Mount (Mt. 5-6) and the sermon and parables delivered at the Mount of Olives (Mt. 24-25).