After hearing what Grindelwald said, Voldemort rarely showed any anger, but raised his wand and prepared to hit Grindelwald with Avada Kedavra. As long as he kills the guy in front of him, the death stick will belong to him, and killing happens to be one of the tricks Voldemort is good at. As for his defeat at the hands of Grindelwald, Voldemort never thought about this because Grindelwald was too old.