When Dean became a demon the thirst for demon blood within Sam grew. He couldn’t bring himself to kill his brother but now he was stuck cleaning up after his messes. Dean still hunted with him, Sam was never quiet sure why. Now he thinks it was all because of this moment. He finally gave in. He craved the intimacy he once had with Dean and now he had it back as well as the itch for his blood finally being scratched. Apparently if all you consume is your demon brother’s blood, you become one yourself. Now, along with his thirst for blood and cravings of sex, he needed someone else’s blood on his hands. Sam needed violence and there was nothing human holding him back. Dean couldn’t be more pleased with his baby brother.