He wanted to argue and say that maybe they were going about all of this the wrong way. Maybe Dean was the one who was supposed to kill her after all. That was what God had said. Dean was 'chosen'. He was heaven's champion, right? And if God thought that he could take on Lilith and win, then why did he halfway begin to think that he stood a chance when God had failed to so much as blink in his direction? Did she know that he was going to fail? Was that why she chose Dean? Despite his desire to argue those points, he knew that he'd get nothing more than eye rolls and sarcastic remarks as far as God was concerned. Ruby wasn't exactly the best person to talk to about religious aspects when she didn't hold any of her own.
Still. There were the other factors consider and Sam knew that he couldn't quite ignore them. The theory was valid. Strong. Very reasonable. But the steps that they were taking to put that process into motion was flawed and he knew it. Asking his father to get possessed by a powerful demon for the sake of a little sideline practice? It wasn't right at all. And yet Ruby was here, pretending that it was a perfectly normal thing for him to consider and that, as it always did, left an unpleasant feeling churning in his stomach.
That was the worst thing about it all. Aside from the obvious; that feeling. He knew he was doing wrong. God and all of heaven would frown upon it, surely. As a religious man, Sam could never turn his back on that particular handful of knowledge. His faith might have been wavering, but it still lived inside of him and, even without it, he easily knew the difference between right and wrong. Heather had gotten so torn up about it that she had actually refused to speak with him the other night because he told her that he couldn't turn away from it. And Dean. He might not have put up as much of an argument as Sam had expected, but he knew that deep down his brother was fuming over the entire ordeal. Sam just didn't want to face his Mom. She would be the worst. He was terrified of the way that she might begin to look at him after everything was finished.
"I never said that," Sam said quietly, "I never said you were using me for anything, Ruby." He turned away from her and raised both hands to clutch at the back of his head. "I'm just worried about this entire exorcism. And...and I don't want to fail." He lowered his arms and, without turning to face her again, said in a resigned voice, "I'm pretty sure it has something to do with you actually caring."