He had to kill the evil monster knowing that would put himself, Django and Hildi in danger? I`m sorry but I don`t buy that nor I buy QT`s explanation that he had to kill Candie because he couldn`t stand to be bested. Schultz was never driven by pride in such amount that it would prompt him to do something stupid. Also, he was flashing back to dogs tearing the man but that doesn`t mean he couldn`t play the part, shake hands, leave, lie low and then serve a cold revenge. That would be in character but I guess that a) would make the movie 4 hours long and Django would be still a co-lead as opposed to finally coming on his own. So it was a plot device that had to clear path for Django`s rise as the sole hero and it felt like a plot device as opposed to something that happened organically.