[Time to go hardcore conspiracy theory here.]

To save the Phantom? Perhaps the Poison was Steven's idea. He stayed in the kitchen to speak with the Phantom, maybe even try to reason with him before killing him? When it became clear what was happening, the Wizard stepped in?

When I spoke to the Phantom, he said that punishment for breaking rules is typically death. What if that's all this is? Steven attempted to end the game directly, and paid the price.