Just let the player do whatever he wants. Why is everybody rolling on their back for a couple of pixels on the screen. Its like , killing a 18/21 years old male human is okay, but killing a teenagers goblin isn't, are you for real? Also, its okay to kill a village, but its not okay to kill some NPC because they appears younger? Uh? What about gnome and dwarf , then? They may be as small as children, but you'd happily slaughter them just because they have a better armor class, an axe, and lived a few more years.
I agree, people should play the game however they want. The problem is that said people want Larian to change the game to their liking and completely disregard what others might like. I've seen threads where one person wants the gore and nudity removed so she can play with her kids. When this is game is rated MA. Another thread wants Larian to remove the scene where Astarion tries to feed on your character because to her it's glorifying sexual assault and it's offensive. Some of these threads nitpick the game too much, it's ridiculous and takes the fun away from playing these game. I hope Larian continues to make the game the way they want to make it. People need to keep their own personal ideologies to themselves instead to forcing it on a video game.
If you wouldn't like killing the tieflings then don't kill the tieflings.
Making all NPCs killable doesn't ruin the game for you.
It has nothing to with this being a D&D game or what the DM says when it is simply political censorship as with most such video games.