He would not kill people who had done nothing to wrong him senselessly. But having a few more spell-forged weapons on hand wouldn't hurt. And the Ironborn were far from innocent and had much to answer for. In the end, his hesitation was short. He might not get such a chance anytime soon, not to mention that it was a local tradition. It was simply the way of life to kill, loot, pillage, rape, and enslave here. And Jon didn't mind respecting the local traditions. After all, when in Rome, do as the Romans do.