Sicarius

When Dean's nine he's kidnapped. He's trained as an assassin, part of "Sicarius", he learns every way to kill a man whether it be with poison, a weapon or his bare hands. The amulet that his little brother gave him is his only anchor in the following years.
Sam is left dealing with the guilt of that night when he was oh so young, how his brother had protected him and the results of that.

(This is Destiel and there is also lots of in-depth descriptions concerning the points mentioned above, just for the heads up. If that kind of stuff upsets you please don't read this- the last thing I want to do is upset anyone (story line wise exempt!))