"Dean you need to calm down I already told you this isn't you the mark is controlling you why can't you see it." Castiel looks towards Dean.

"Please just don't let it control you, I don't wanna see you get hurt Dean you mean to much to me."

To me, the worst ending for SPN would be for the boys to ride off into the sunset, ready for their next hunt. It may seem like a happy ending but it’s not. It’s a horrible ending for two men who, deep down, do not like hunting. But do it because they are good at it and that’s all they know. Seriously, that is all they know. And that is what makes them so heartbreaking. Dean thinks of himself as an attack dog. In season three, he feared dying because he was so sure that if he did, he would return as a Demon because of all the hunting he did. And by the end of the ninth season, it turned out, he was right. But despite that, he kept on hunting afterward. 

So, I want the boys to have peace. And the only way for them to have that is for them to die. Just wipe them off the board, dead. 

No driving off into the sunset. No flashforward with a family and kids. Because in that future, the potential of being dragged back into fights is always there. Just that sweet, sweet conclusion of death. No Empty. No Heaven. No Hell. Just oblivion where no one can touch them.