Well I gave the example from the D:OS games, where the claim is you can kill any NPC without hurting your ability to complete the game or any quests.
No, the claim is that the main plot is completable if you kill most NPCs. Side quests can easily be broken that way, merchants can refuse to deal with you, etc, and it can be much more difficult to find required information for main plot quests.
In D:OS there are consequences for stealing if you are caught. D:OS 2 extended that so NPCs would search for the culprit when items were noticed missing, and guards could search character inventories (with a time limit). If you wanted to be completely realistic and immersive there would be much less random loot, and more than one or two missing items shortly after new people show up in town would have everyone assuming you were thieves, and depending on the setting, not necessarily shy about treating you that way.
