He shoots Candy because Candy is such a repulsive man and the horrible things he has done to others. Not for the way Candy is treating or belittling him personally. Schultz is a man for who racism is kind of comically dumb. He's divorced from it, so when he sees the real face of it he doesn't know how to react.

Why did Schultz die in Django?

King Schultz (early 1800s - May 5, 1859) was a dentist-turned-bounty-hunter who freed Django and helped him rescue his wife. Schultz was eventually killed in a shootout when he shot Calvin Candie in the chest with his ladygun.

Is Calvin Candie sleeping with his sister?

Lara Lee Candie-Fitzwilly is the widowed sister of Calvin Candie with whom he shares an excessively affectionate relationship, bordering on incestuous. After Calvin's death, Lara conspires with Stephen to send Django to a mining company to work him to death as revenge for her brother's slaying.