Dean was harboring bad thoughts. He had been for as long as he could remember. Mostly they were just glimpses of violent ideas, but sometimes they were more defined. The first one he could clearly remember happened when he was about twelve.
He and Caleb and Dad and Sammy had been sitting outside on Caleb's porch. Dean sat at a table, cleaning off his and Dad's knives. Dad and Caleb sat on the steps, having a beer and Sammy sat on the porch with his crayons, drawing.
Dean watched the peaceful scene as he polished the blade in front of him. Dad looked relaxed for once, and Sammy's face glowed with an unusual happiness. Caleb turned and smiled at Dean. Then it happened. He suddenly wanted to grab the knife that he was holding and slash the smile right off of Caleb's face. He wanted to rip and cut that throat right out. Then he wanted to cut off his father's hands, the hands that had pushed him into so many hunts, and slice off the tongue that had commanded him everywhere. And he wanted to slash himself, put himself out of this crazy misery, with all the red iron smell of blood chasing him. He shook himself slightly and gave Caleb a tight smile in return and turned away, staring at his hand, which had gone white from gripping the knife in his hand so hard.
That had been the first extremely defined bad thought, but it definitely hadn't been the last. As he grew up, they became more frequent, more violent and more defined. He got these thoughts out of his system by killing whatever evil thing they could find. But that hadn't stopped them at all.
When Sam had left, Dean had grabbed the keys to the Impala and driven, fast and hard, with loud music blaring, trying to drown out the deadly thoughts. The thoughts of strangling Dad with his own bare hands for yelling at Sam, the thoughts of hurting Sam so bad he'd have to stay.
And he'd stared at Sam and Jessica when he'd come late at night to tell Sam of Dad's disappearance. He had watched his brother's comfort with her, and had had the sudden thought of bashing both their heads in for being so stupid, taking special care with Sam for wanting a life he knew was an illusion, but he pushed it away. He'd needed to think clearly in order to convince Sam to come with him. And Sam came with him. But there had been more thoughts along the way.
Now he stood in the crowd of people at the carnival, despairing. His father was dead, his brother was annoying him, the dumbasses at the freak show here had railed on him for nothing, and he was pissed off. He stood in the center of all these happy families, seeing red. He wanted nothing more than to rip their heads off, slash their bodies up into little pieces and tear their laughing squealing little children to shreds.
Sam's figure came into view, and Dean suddenly felt the urge to pull out the knife hidden in his jacket and punish his brother for being such a jackass. But then Sam put a hand on his arm, his touch sending signals of 'I want to comfort and help' and 'Are you okay?' and he flinched away from the contact. He couldn't stop these wild, violent thoughts. But Sam couldn't help him. He would just have to help himself. They just weren't healthy.
In the end, the thoughts became too much. Sam wanted to help, had pretty much said it point blank. Dean had refused. It resulted in Dean's beating up the Impala, but Dean really didn't want to hurt the only living comfort he had left. And the thoughts were terribly familiar and too precious to give up. Violence like this was what kept him alive. And violence like this was why he was dying so very fast.