While utterly opposed to the idea of killing a Mage's familiar when within the city for the sole reason of it being there, I do wonder:

When given a warning or making it clear you're about to attack the familiar, would a DM be (absolutely) required to be around when in front of NPCs?

Mostly just curious.

Yes, it is still mostly considered pvp, and, the familiars have a habbit of bugging factions, so it's entirely possible that the bystanders will become hostile to you.