'Its what my world would do' is the GM version of this excuse, and it's just as bad. If a player can decide that their character acts differently in order to respect the social contract, so too can a GM have their NPCs or setting as a whole act differently.

Whether or not background NPCs being harmed is against the expectations of people at the table is going to be a table-specific consideration. And it may be unspoken and therefore seen differently by different people at the same table. So that's why it needs to be discussed from an OOC frame of mind.