"You need the blood of your father," Balthazar informed Sam.
"My father's been dead for years." Sam was frustrated. Maybe Balthazar could resurrect his father so that he could kill him. He really wanted to keep his soul out.
"You need the blood of your father, but your father needn't be blood. " He knew that if Sam killed Bobby, that would be the last straw for Dean. Especially since resouling him would no longer be an option. Then Sam would be totally at his disposal. Sam would be a great asset to his business.
Sam took a minute to figure out what Balthazar was saying. Bobby had been like a father to him. Dean would be mad, but he would get over it. Sam had to do what he had to do to survive.