Arthur was not a naturally mean nor aggressive person; however he was short on patience and angry that someone wanted into his subconscious this badly. "A possibility doesn't make a reality. Once the choice is made, then it becomes reality, and before that, it is just potential. They're not the same thing. There's no such thing as time travel, and no such thing as another reality. There is only one."
He shifted on the expensive leather soles of his shoes, and moved away from her onto the next glass display case. "You can experiment and push limits without breaking out of reality. Death breaks you out of reality, and after death, there's no guarantee of anything." Maybe the gun wasn't the way to go. If he wanted to draw someone out, a long fall would be better.