In episode 15 A man named Jeffery adopts a dog and later kills her. They don't show the dog die but you can see Jeffery's shadow when he kills her and hear her yelp.
not a dog fighting scene, but in episode 4 a minor character is mentioned to have organised dog fights in the past (and then died because of the guilt)
S7E2 SPOILERS While sam is in a warehouse at the end of episode two and dean is preaching about his hallucinations, he presses on a wound on his hand until it bleeds to make them go away. he does this on purpose and for the pain. It does get lucifer to go away, however ;)