Dean Winchester always knew how he would die. In combat, on the battlefield. Just like the good little soldier that he was, that John had wanted him to be. He knew how he wanted to go, but, admittedly, he never thought that it would end up as suicide. If you had told Dean three years ago that he would kill himself, he would have punched you in the nuts, then gone out to a bar, to, undoubtedly, flirt with some hot bartender. Then again, if you had told him that Sam wouldn't want to be his brother because Dean tried to save him, he wouldn't have believed you then, either. His world was upside down, he missed Cas, and he couldn't see a way out. In all honesty, who could blame him for just wanting an easy way out. Mom was dead, Dad was dead, Ash was dead, Ellen and Jo were dead, Bobby was dead, and Kevin, the poor kid, was dead too. Because of Dean. People don't really know how to live with guilt. They just push it down, and avoid thinking about it until they can't anymore. Hell, Dean's killed more people than he can count. The monsters, the demon meat suits, the innocent humans. None of them deserved it. He did.
Although he would never admit it, Sam's rejection had driven the final nail into his coffin, metaphorically speaking. It hurt him to see the one person who he was sure that he loved turn his back. It hurt ever more than the time Sam had left for Stanford. He was just so tired of it all. He just wanted it to end. Dean picked up the colt in his hand, a poetic end, considering that he was the real monster. With steady hands, he opened the door of the impala, and drove until he didn't know where he was.
"Goodnight Sammy," he mumbled, putting the gun to his head and pulling the trigger. Dean liked to pretend that he was so high and mighty, but he was really nothing more than what he hunted.