In my opinion it's the most important scene in whole movie. It explain everything which was before it and after. Remember the scene, where Schultz is telling Django legend about Broomhilda? Django can only become Sigfrid if he save his love by himself. He need to get at the top of the mountain, cope with fire and kill the dragon. Schultz teach him about shooting and help to reach Candyland (top of the mountain), but rest of the task was up to Django. If they left with handshake, Schultz would rescue Broomhilda, not his ex-slave friend. Going further, Schultz despised slavery, racism and all those dark things represented by America at the time. Candy was a personification of all of those terrible things. And by handshake he would agree with this. When it comes to Schultz's occupation, he was dealing with killing people and living his way of life. But when Django and him flip and roles, he was unable to be cope with playing a slave trader. He could save poor d'Artagnan, but was stopped by Django. All those things have accumulated in him, so the only thing he could do was killing Candy, as his voice against racism and slavery. IMO this scene is brilliant and can be analyzed on so many levels. Without this part, "Django" won't be so great movie.