Castiel could kill; he could feel the cool of the blade and the hot sticky blood as a life force ended in his hands. He could be merciless, when he had to. He could fight; putting his body and life on the line time and time again, if it meant that others were safe. He could doubt; though once upon a time he couldn't.
Cas could pretend that he didn't know; could turn the other cheek, shift his eyes away, rub at the back of his neck every time Dean stared at him with that lust filled gaze. He could pretend that he couldn't read Dean's thoughts and feel every feeling he felt. Every high, every low, every fear, every doubt, played along Castiel's conscience every time his hunter was near.
Cas could shut other human's out; he could ignore their thoughts, their emotions, their prayers. He could close his heart and his mind; but he couldn't pretend to ignore Dean. The hunter cried, and the angel came running. Castiel couldn't deny that he was caught forever in the constant ebb and flow between lust and love and insecurity.
Castiel had tried, he had tried so hard to shut his hunter out. He tried so hard to ignore the hunger he felt for the human; the love that threatened to overwhelm his very being. The harder he fought, the further he fell. When Dean prayed to him, it was his sirens song, and he always went running.
Cas couldn't forget the horror of seeing Dean with black eyes; his fear when Dean towered above him, angel blade in hand. He had known for months, maybe years that the hunter had felt something for him beyond what he had admitted out loud; perhaps even in his own mind. The moment the blade struck a book and not his heart, Cas knew for certain, he knew his love was returned. Whatever part of Dean that still existed, it couldn't kill the man he loved. Not even the Mark of Cain could make Dean kill him, and Castiel knew that. Only love could overcome something so ancient and dark.
He couldn't pretend that seeing him alive when he was so sure he had really done it this time; when he was so sure he had seen his love for the last time, hadn't nearly undone him. Only realizing that Mary Winchester had returned from the dead had kept the angel from falling apart entirely; the magnitude of her return far more pressing than his overwhelming relief.
Castiel couldn't deny that for far too long, he had loved the older Winchester. It wasn't that angel's were incapable of love; on the contrary. Angel's loved perhaps too well; so well it consumed them from the inside out. They protected their hearts with a stoic outer shell because when they did love, they went mad with it.
He couldn't pinpoint the exact moment he realized he loved the human. Perhaps it was the moment he had ripped him from hell, leaving his mark behind. Perhaps it was when Dean foolishly plunged an angel blade into his heart.
Perhaps it was sitting on a park bench, where Cas admitted out loud such treason that should he have been overheard, he would have been put to death without trial-he had admitted to doubting God. It was the first time he had rebelled, but certainly not the last. Dean had inspired that in him, and for that he would be forever in the hunters debt.
Castiel couldn't forgive himself for Lucifer. He knew, ultimately, that releasing Lucifer was a mistake. He had thought for a brief moment that he was useless; easily disposed of and forgotten. Knowing that Dean fought so hard to keep him and his vessel safe had touched him to his very core.
He couldn't deny that the Winchester had stolen his heart, and that he would never be so fiercely loyal to anything again in his life.
Castiel also knew, deep down inside, that he couldn't ever love the hunter the way he wanted to.
He knew that Dean WInchester could never know he knew; that every thought and fear and doubt he had reverberated through the angel, making him love the hunter even more.
He knew that his Dean would never let love in that way, he knew he had to watch from afar, and love him the way Dean would allow. He knew he had to be cautious in his love, because someday, eventually, Dean would die. He was a human, whether it was now, or a year from now or when he was old and wrinkled; Dean Winchester was going to die.
Castiel was not.
He could ignore the voice in the back of his mind that whispered, "love him anyways, love him anyways." At least for a moment, he could ignore it. He could put it aside, knowing deep down that the hunter was not ready for his love.
Castiel didn't know if he could ever face the loss of his love, though he knew eventually someday he would, and the thought ate at him more than anything he had ever experienced before. More than the thousands he had killed, the angels, the humans, the demons. He had faced countless wars, feasts, famine's, plagues, the end and beginning of new ages... nothing in his existence scared him more than the knowledge that someday his hunter would die, and Castiel couldn't ever follow him where he went.