Stein smiles his evil smile, before standing up, taking one last sip of the hellish coffee that Bob specializes in. "Very well. I'll kill you one of these days, of course." It's not stated as a boast, or a challenge, but as a fact. 
Bob smiles as Stein states this. "I've survived for over a billion years now. I plan to stick around for the next billion." He stands as well, never turning his back to Stein, with an attittude of almost eagerness at the thought of Stein's attempt to kill him. Stein struck not through overpowering his foes, but through a battle of wits, leading them into traps and tricks, like a game of chess with lives on the line. This was how he defeated beings far more powerful than himself. "However, I look forward to the day you try. It will be most interesting indeed, after all the stories I've heard."
Without saying another word, the God-Killer turns, at ease, knowing Bob would not attack him. He glances at Mithias, completely ignoring Hunger, and then steps into the purple vortex that leads to his lab, disappearing from the scene.