There was definitely a bit of a pride factor there. They had gone through this whole brilliant charade to gain Brunhilde's legal freedom, but it was turned sideways right in its final moments. Not to mention, it wasn't even Calvin Candie who had uncovered the plot; it was Stephen, who only really figured it out through a hunch and later interrogation. Calvin, however, had such an aggressive superiority complex that he took full credit for uncovering the plot, and attributed it to being smarter than Schultz. The combination of Calvin Candie being such a disgusting person, the uncovering of the plot, and the heat of the moment is what caused Schultz to be unable to stop himself from killing him.