Pretty sure that baby murder is not a "21st century liberal" thing.
Actually, it pretty much is when you are talking non-human species.
For hundreds and thousands of years, people would kill baby wolves just as easily as they killed adult wolves. There was very little thought process of "aw, the poor little things". Same with coyotes and other predator babies. Sure, some people might try to domestic a baby wolf. But there was no moral ambiguity about killing them.
Ditto for a lot of species. The Bible gave mankind dominion over the earth and its animals, and that was morally ok for millions of people for thousands of years.
Even today, if you go outside and walk on your lawn, you will probably kill hundreds of insects including baby insects. It does not morally enter most people's mind that they are slaughtering these creatures.
But from a strictly 21st century moral POV, that's a form of murder. Nobody talks about it, because "they are only insects". The morality line tends to shift based on human culture, education, and social needs. Just like most PCs don't talk about the fact that their job is:
1) Breaking and Entering
2) Murder
3) Theft
because it's a game.
D&D is also not a medieval Earth game. It features magical robots, air ships, lizards that are smarter than humans, and angels and demons you can play poker with.
For the most part, it is. Technology is pretty much limited to plate mail, swords, castles, etc.
Yes, there are "magic is technology" worlds like Eberron, but that tends to be more of an exception than a rule. And there are intelligent species other than people, but society is still basically hunter/gatherer combined with agriculture, mostly in medieval-like settings.
The reason why food source dictates morality is that in the 21st century, most nations on the planet have people go to a little store and trade paper for food. It is almost always there. Most people do not have to slaughter animals to get meat, or to protect a farm from predators.
But when one lives in a medieval-like setting where wolves and worgs exist, that food only comes into the town if the farmers (or adventurers) go out and kill baby wolves, baby Orcs, and a lot of other baby predators (along with mostly adult predators). Morality does not really enter the picture when one is talking survival. If your deity is the enemy of the Orc deity, no moral ambiguity about killing any sort of Orc at all.
A given player might add that morality for his PC, but it is not required (unless maybe the DM requires it).
I agree that it is a bit myopic to make monstrous humanoid infants a monster type, but in all fairness, 5E is aiming for the old days, which were built heavily on fantasy racism and little thought put into what being a murder hobo really means from an ethical standpoint.
As a DM, I might just use these creatures specifically to put some massive guilt down on the PCs, and to basically put them in the shoes of being the real monsters.
Overall, I agree that WotC really should keep the problematic nature of these sorts of depictions in mind, and to avoid having infant forms of intelligent monsters in this way. It may not be the most vile of acts to create such a bit of fiction, but there are many other ideas available which are less prone to problems.
Agreed. But it goes back to the demon worshiping thing. Just because D&D had demons in it did not mean that players worshiped demons. It meant that the concept offended some people's 20th century religious morality. The same with this. Just because the 5E D&D has baby evil intelligent creatures in it does not mean that players actually find baby killing to be morally ok. It just means that it is an aspect of the game that is available and potentially offensive to some player's real world morality. No more, no less.
Worth keeping in mind that "it's a game" isn't a defense any more than "it's a book" or "it's a conversation." Games are expressions of ideas, along with their mechanics. One does not automatically agree with the implications of their art (good people can write stories about evil people!), but it does still express ideas, and offers them up for criticism.
Why does there need to be a defense? There is nothing wrong morally with killing fictional infant creatures.
All morality is subjective. There are cultures in the world today which feel that it is morally ok to kill whales. If you feel that it is morally wrong to kill whales, that does not make you morally superior to them, nor does it make you right and them wrong.
Morality is subjective. It's based off of cultural needs and desires. In a D&D game, it's based off of fictional cultural needs and desires.