Sam wanted to argue that you don't sacrifice the people whom you're supposed to protect, no matter how important the cause. You didn't treat people like pawns--and you never, ever took advantage of young, adoring apprentices who would do anything for you, no matter that the cost to themselves. It was what separated the good guys from the bad guys.
But he also knew that while he had only sacrificed himself, his brother Dean had had to watch, and live with the weight of the knowledge that there was probably no coming back from where Sam had gone. And it was never easy, no matter what side of the equation you stood on. So he held his peace.
"Molly, I told you, your council would have my head if they knew half the things I've done, trying to save the world. Why would I go around blabbing anything?"