Disclaimer: I don't own a thing. A.N. Today's prompt was about favorite endings/season finals, and well. This happened.
Makes you wonder
Later (way, way later – at the moment, they're kinda busy) Dean will wonder if Death had technically committed suicide. All the pizza and delicious street food in the world might not be enough to keep him going, especially if he considers people as bacteria, Lucifer as a bratty kid, and god...well, just another client (eventually). It must be damn lonely, his employees notwithstanding. He didn't look like they were having office parties, anyway.
Sure, Dean approached him first. If anyone had the power and the knowledge to handle the Mark of Cain, it had to be him. Dean got this close to killing Cas. Something needed to be done, before he lost control completely and...well, who knows. He might have mowed down the entire planet. Surely, Death would want to stop it, if only to avoid extra work.
At that moment, Death's reasoning had even made sense. Remove Dean to the other end of the universe (a deserted one), where he couldn't hurt anyone anymore... But Sammy had to be taken out. The kid hadn't let him go when he was a demon, for God's sake. There was no way that he wouldn't have researched and schemed and fought till he found a way to undo their elegant solution. In fact, his baby brother was way further along with that than anyone suspected.
But...asking him to kill Sammy. Him, of all people. As if he wasn't a fucking frequent flier on Death's services, most of the time for his brother's sake. As if he'd ever asked Death favors (surely, that kind of mileage came with perks, even when you were a bacterium) always, only for Sammy. (That time was about stopping Cas, so it really didn't count, did it?) As if, when Dean had to make a choice, to Death's face or not, he hadn't picked Sam each and every time. It didn't take much to see a pattern, did it? No matter what was hanging in the balance.
It's not like they didn't have the chance to slam hell closed and – just imagine. No more demons destroying families like Yellow Eyes did to theirs. No more hellhounds dragging people down after an unwise contract (and he knew fisthand how that felt). No more – so much torment and things he'd never have the words to say, for anyone. But losing Sammy? Fuck no. Not acceptable. And sure, he'd regretted it at that point – thought that they weren't worth it. That it was a stupid, evil move, but it's not like his soul had changed, deep down, no matter what shadow the Darkness was casting, and how much loathing it dredged up. For everything he – either of them – had done and not done and just...been.
Maybe, if Sam had fought him to the end, Death would have won with his stated plan. Lost what he really wanted, maybe? The Mark liked that. It couldn't wait. But – whether because his brother was that smart, or because he had no goddamn sense sometimes – Sam had given up struggling.
What he hadn't abandoned was his faith in Dean, in his brother being good, at his core and eventually. Because he knew his plan was ongoing? Did he fucking plan to let Dean kill him and then, someday, maybe someday soon, he'd be free of the Mark and...what? What did he expect Dean to do then? Christ, Sammy.
God might have disappointed Sam (though, for all he knew, Sammy stil prayed every day...not like he'd realized the first time around), but Dean? No matter what, Sam refused to stop loving his brother, however much of a monster he turned into, including to him. Sam refused to stop believing in him, even when he really shouldn't. Whether then or back when Dean managed to con him into thinking the Easter Bunny was just another creature way too long (eleven and a half, jeez).
Was anyone surprised that Dean couldn't make himself disappoint the kid, much less kill him? Did anyone really expect the scythe to swing anywhere else? Death had been close enough to take down. He should know the thing's reach better than anyone.
Whoever gave Dean a weapon and asked him to use it on Sammy had to be an utter idiot or suicidal...or both.