Sam knew he had some problems. He knew what he was. He needed to tell someone, anyone. But he couldn't, he knew he had to keep it a secret for everyone's sake. He couldn't just tell someone, "Oh yeah, I hunt Demons and monsters with my big Brother." That's just messed up. He wanted to do so many things though. He wanted to talk to Dean, he wanted to Propose to Jess, but it's not like he can do that now.
Of course since Jess died he had been going on hunts with Dean. Before Jess died, Sam had been getting nightmares, or… Visions. Dean used to question him about it, but now he doesn't even care. He's too caught up in being buds with Cas to notice Sam. He doesn't notice that it hurts him.
Everyone started to take a disliking to Sam after he began drinking Demon blood. He never wanted it to be like this. The burden came to him. Sam asks himself, 'Why can't they accept that I won't ever be the same?' He's already faced up to what he is, a monster.
He tries to control it, but the thirst keeps getting stronger. He needs demon blood. He wants to fight it, but he can't. It dominates him, it commands him. Do it Sam, It says, drink the blood. Kill innocent people, Sammy. He decided he couldn't stand the pressure anymore. He searched the random piles of stuff his safe, and found the colt. He wrote a note to Dean.
You were right. I'm not okay. I am prepared to do what ordinary people won't do. You want me to shake hands with you in hell? I won't disappoint you. I may be on the side of the angels, but don't think for one second that I am one of them. This is my note. That's what people do, don't they? Leave a note? Well, I suppose this is goodbye Dean. Say hi to Cas for me. ~Sam
Sam took the colt in a shaky hand, and pressed it to the side of his head. "Goodbye Dean."
FIN
(The note was different quotes from The Reichenbach Fall -Sherlock)