THEN King Arthur let send for all the children born on May-day, begotten of lords and born of ladies; for Merlin told King Arthur that he that should destroy him should be born on May-day, wherefore he sent for them all, upon pain of death; and so there were found many lords' sons, and all were sent unto the king, and so was Mordred sent by King Lot's wife, and all were put in a ship to the sea, and some were four weeks old, and some less. And so by fortune the ship drave unto a castle, and was all to-riven, and destroyed the most part, save that Mordred was cast up, and a good man found him, and nourished him till he was fourteen year old, and then he brought him to the court, as it rehearseth afterward, toward the end of the Death of Arthur. So many lords and barons of this realm were displeased, for their children were so lost, and many put the wite on Merlin more than on Arthur; so what for dread and for love, they held their peace.I've always read this part of Malory's tale as being a reiteration of the Biblical story, and a rather implausible one. I'm not sure whether a Jewish king, with the backing of Roman soldiers, might have really carried out such an order. Perhaps the Romans would have been willing to sustain him on his throne against the outrage it would merit, as it would weaken and divide a subject population. Perhaps the kingdom was at that point so inflected by the kind of tyrannies common to that region in that time of the world that it was not out of order.
It is just the sort of thing, though, that the British tradition in Malory's time could not have accepted. Kings would and did ask for hostages from powerful families at times, as a guarantee of good behavior. But they treated them well and raised them so long as good behavior was assured. To have killed the hostages, not of one family but of many across the kingdom, would have brought open war from the nobility against the king. Certainly in Malory's own day the Wars of the Roses -- in which he had some part -- showed that the dignity of royalty was not unlimited if they abused their authority.
In any case today we are supposed to reflect on them, and all innocents killed for someone else's purposes, convenience, or benefit. It would seem we have quite a score of our own to admit to on this day.