During the climax when Candie realizes his guests are plotting against him and he slams the table in anger, actor DiCaprio accidentally cuts his hand on glass and doesn't break character. Smearing the blood on Brumhilda's face was totally improvised.
Tarantino films contain many examples of villains and anti-heroes who, despite being cold-blooded killers, possess consistent codes of ethics and show humanity at the most bizarre moments. Schultz agreed to help Django to rescue Brumhilda because he was so impressed with Django's help in tracking the Brittle brothers early on (remember when he calls him Siegfried?), and his hatred of the institution of slavery drives him to shoot Candie instead of just shaking his hand, walking away, and cutting his losses.
Such outlaw morality works perfectly well in Django Unchained
because abolitionists were considered utterly villainous even in the North where slavery was illegal for sociopolitical (not moral!) reasons. The protagonists in classic Westerns typically defend a society in which have no real stake; with Django, Tarantino does unto this conventional formula as Marx did unto Hegel's dialectic.
I'm afraid Sab's stupidity is completely sincere.