Factual error: When Django and Schulz ambush a group, they kill them with their Henry repeating rifles. That rifle was introduced 2 years later, in 1860.
Other mistake: When Calvin sits down to eat his white cake, the cake is a new slice. When the camera switches and comes back, it is almost gone. When the scene switches again, he is eating a full piece of cake again.
Continuity mistake: When Dr. Schultz shoots Candie, in the 'reaction' shot where it zooms in on Stephen screaming, Candie's bodyguard (the one with the mustache and bowler hat) has already turned around to see what just happened. In the very next shot he now has his back to the camera and then, a second later, we see him slowly turn around again to see what just happened.
Continuity mistake: After Calvin cuts his hand and forces Broomhilda to sit down, Stephen places his hand around her neck. A shot later it’s on her back.
Answer: Django and Schultz killed some of Big Daddy's farmhands, and he wasn't going to let a freed slave kill some of his workers. Schultz knew Big Daddy would come after them for what they did. Plus Django would never let go of a chance to kill a slave owner who was trying to kill him.