Based on this from ectoimp‘s stream, and the conversation that went with it. Hope it sounds good. Biggest warning is that Lewis is an Ass with a capital A. But at least he’s above the whole murder thing (now, at least).
He didn’t know what exactly possessed him to do it.
Usually, it would have never even occurred to him; Mystery was an ever-watchful presence, eyes on both Vivi and Arthur in case he pulled something.
But tonight, on the outskirts of an unfamiliar town and in a potentially dangerous neighborhood, Mystery had let his nerves reach him, and when Vivi had left for groceries, he walked by her side.
He had trusted that Lewis would do nothing to Arthur while Vivi cared for him, before proof that Arthur had “betrayed” him had been found. And if anything dangerous lurked, several cases proved that Lewis would reluctantly come to Arthur’s rescue, if only because if anyone was to exact “justice” on Arthur, it would be him.