This is based on an awesome piece of fan art i came across: I do not own it but credit to the artist :)

Link:

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Never had he felt so dominant. No weaknesses, no vulnerability just the raw power coursing through him. He could feel it absorbing him as he knelt there on the warehouse ground. Kneeling in a pool of Abaddon's spilled life force, still warm as the pleasant liquid soaked into his clothes. The blade was in his hand, completing the Mark and giving him utter control over the power he had just unleashed on the last existing Knight of Hell.
He was breathing heavy, panting. The Mark of Cain searing into his skin and emanating a thick gleam through his sleeve. He closed his eyes for a second, taking control of his breath, willing it to slow.

All of a sudden the pain was gone and he was left in an eerily calm silence inside his own mind. Instinctively he opened his eyes and drew the blade closer to his chest. His other hand tightening amongst the thin strands of red hair. Abaddon's head rests on his leg. The severed flesh of her neck bleeding out. Not that it mattered. She was dead and Dean had been the one to do it. No mercy had been given and no regrets surfaced.

Sammy had doubted him. Doubted his self control. His brother was mistaken and Dean had proven it. Sam of course wasn't anywhere in sight. Dean had given him the slip. Sent him and Castiel on a false trail while he followed the right one, confident and filled with anger. He craved the battle that followed. Wanted to look into his enemies eyes before he separated her pretty little neck from her pretty little body. That's exactly what he had done. Right after giving her false hope that she would triumph, he had delivered that final swing with the First Blade tight in his grip, it's reassuring weight daring him to do it. To finish and fulfill the mission.

Dean had recognized the change, he had acknowledged it long before his brother and the angel. It changes the way he used to feel. The blood-lust, the way he savored the hunt even more so than he used to years ago. It had scared him at first and forced him to turn to alcohol. He was supposed to be the hero and the savior but he knew that ship had sunk a long time ago. He had past blood on his hands, from friends. From family. He was guilty of a whole lot more than anyone could imagine.

Despite his continuous suffering he would never truly compensate for his faults. He knew that and had willingly accepted it. He is no better than the things he hunts, he is a monster. Someone who didn't deserve to be saved.

He deserved to burn.

His train of thought was broken when a deep hunger arose inside of him. Not the normal, natural hunger but one far more dangerous. Yes, he had won the battle and he was relishing in the pride and satisfaction. But he needed more. He loved the sensation of his victory so much so that the craving to taste it only grew stronger with each passing second.

He looked down at his left hand, it was soaked in blood as well as the rest of him. There was no doubt in his mind that he had succeeded as he glimpsed the ever growing stain of blood beneath him and the decapitated head resting easy on his thigh. He was staring at the First Blade, his knuckles white with the force of grip.

It was a sadist move, a sadistic act that made the oldest Winchester grin with anticipation as he brought the First blade towards his waiting mouth. The cooling blood streaming down the jagged blade onto his hand and down his arm. He raised the handle upside his face, the blade just centimeters away from his chapped lips. He consciously allowed his tongue to run along the edge of the dagger tasting the sweet, iron-tainted liquid.

There was a blatant bang as the rusted metal door to the warehouse swung clear off it's hinges. Dean didn't bother glancing up, he figured his brother would follow him. Though it was a little late timing. It was already over.

He heard it clearly as the younger male drew in a sharp breath at the sight of his big brother. He saw the unconscious step back and fear in his eyes as Dean made eye contact with the man that he had once protected with his life. A Malicious expression along with a desperate hunger he needed to fill contorted his features.

He knew one thing for sure.

He wasn't the righteous man any more.

He never was.