Deep down, he always knew it would kill him. I mean, how could it not? It was the freakin' Demon Trials, designed to slam the gates of Hell and take all those black-eyed scumbags with it. There's always a price to these things, and goddamnit, he was going to pay it Maybe this time, he thought, Dean will finally be proud.
It was like the stars aligned on this one. Here he was, the boy handpicked by Azazel, who fell to the temptation of the demon blood, who turned his back on his own brother for the sake of a demon, only to release Lucifer from Hell. His whole life has been plagued and destroyed by demons. So isn't it fitting that he gives his life to finally put them back where they belong?
It wasn't as if Dean needed him anyways. He would always just be the disappointment who couldn't be trusted, not even to search for his own brother. No, Dean would be okay. He had his Angel, his charm and good looks. Hell, he was able to find a brother in a vampire, so why in god's name would Dean need him?
But then again, this was Dean. Dean, who gave him the last bowl of Lucky Charms; who traded his soul to give him back his life; who would give up his own freedom to save his humanity; who ended up sacrificing the one person who never let him down just to get him back from Purgatory; who never stopped believing that his little brother was the most important thing in the world. And he was right; past or present, at the end of the day, nothing comes before his Sammy. And that was all he needed.