Ah, I find that time travel stories work so seldom, yet you pull it off so nicely here. I could actually picture a whole verse based on this oneshot. *wink wink*
And the angst, this marked, conscience-smitten other Sam about broke my heart into hundreds of pieces. Kind of makes you wonder what awful things he did and what happened to him, to drive him to go to such lenghts to right the wrongs. Great work!