The quest states that night after night, s/he comes back. So, by stopping them one night, you're only delaying the slaughter.
What happen if you just let it keep going then? I've always done that quest the day I got it, but what if I go there every midnight, watch the Nightkin make swiss cheese out of a brahmin, come back the next night and repeat? Do they just keep getting more brahmin? Does the quest fail? Does the Nightkin find Brahmins somewhere else to shoot?