i think the dog scene showed that the doc thought he got the brutility that was involved in slavery, but he really didnt. He was a bounry hunter and had no problem killing criminals for the bounty but seeing a man ripped apart by dogs he could not take. Django already knew the brutality of slavery. he had witnessed it before he had experienced it. why would he show as mush distress as the doc? i liked this scene because he showed the a white man thought he got it, but he really didnt until it was in his face. And every event after that made the doctor even more angry. which is why right before shooting candie he said he can't help it. he was so angry and disgusted by the way that man viewed other humans. Django was no stranger to this treatment and attitude. i also think he knew that if he showed too much emotions such as anger or sadness, their cover would had been blown. he was suppose to be use to seeing such horrors in the mandigo fights. he couldnt let something like that fizzle him up.