There is a scene in Django Unchained in which Candie (Leo DiCaprio) demands that Schultz shake his hand.
Did Candie really want just a handshake or did he have something more sinister in mind?
I believe it is to be interpreted as "getting the last laugh", "getting one up", or "getting the better" of the other person. A game of wits in which the two are playing mind games with each other to see who can get the last laugh.
Throughout the scenes leading up to this moment, after Candie forces Schultz to pay 12,000 for Broomhilda, Candie repeatedly says things to Schultz such as:
You're just upset that I got the better of you
(not an exact quote), implying that he outwitted Schultz. So when Schultz refuses to shake Candie's hand, Candie devises a scheme to "get one up" and force Schultz to shake his hand, to which Schultz replies with a gun shot to the chest. Schultz ultimately gets the better of Candie, but does so knowing he will lose his life. "I couldn't resist" shrugs
Anybody think Schultz poisoned Dicaprio? He walks off scene after asking for a pen and then comes back and give Dicaprio the pen when Dr Schultz takes the pen you'll notice he grabs it in a certain spot, as apposed to Dicaprio grabbed it full fisted. I think he poisoned him and was gonna leave but by shaking his hand he would've been poisoned as well. Back against a wall he shot him unexpectedly to end it all.