Other mistake: When Schultz meets the Speck Bros. visible breath comes out of the slaves' mouths due to the cold, but not from Schultz or the brothers'.
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.
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.