Yes, you can. It won't save any lives of the Nipton townsfolk, as the massacre has concluded by the time the player arrives. Plus, you will be in for one hell of a fight as there are six Legion Frumetarii to take down, including Vulpes, and none of these warriors are slouches. This will cause infamy with the Legion for a while, but in the second act of the game, the slate is wiped clean, and you can play through the whole main story arc as the Legion's champion.