Note: Takes place in 4x10. For the specific scene I'm referencing, just watch the video of Dean and Anna kissing before she finally goes away and look at Castiel's face when Dean kisses Anna. This is my version of what went down in poor Cas's head during that unfortunate kiss. This is a cheesy, lame, oneshot, but it wouldn't leave me alone. Inspired by this Buffy line: As hardcore drugs go, human emotions are useless!
When all was said and done, Castiel knew they had to take Anna. While he had no intention of helping her evade Heaven, he had no intention of killing her himself. They had a history. He had served under her for hundreds of years and she had always been a consummate and faithful leader. He felt a certain fondness for her and seeing how far she had fallen, ripping out her grace, becoming human willingly, well, it touched something inside of him. He felt almost melancholic when he thought about her fate. He pitied her and was sorry that she would have to die. He hardly understood why she was choosing death. He didn't understand how she could choose humanity, a flesh prison, over absolute power, immortality, and serving the absolute grace of Heaven. He didn't understand her slavery to human emotion and why she clung to it, even in her last minutes of life. Anna was addicted to human emotion. She craved feelings of elation, lust, violence and guilt. She craved sex, the love of a human family, and the love of a human man. As an angel of the lord, Castiel was detached from emotions and physical need. These were things that Castiel could never understand.
He watched as Anna said something to Dean about forgiving him. Castiel's eyes went to Dean, who looked absolutely sick about the loss of the fallen angel. Castiel felt something tug at where his vessel's heart must be. The look of devastation on Dean's face was doing strange things to him. He suddenly wanted to release Anna, let her lead a human life, let her die a human death, even though just thinking this was tantamount to treason. Castiel had no idea why he suddenly felt empathy for Anna. After all, she was a traitor. Castiel felt waves of loss and guilt coming from Dean, who had brought the angels to this space, essentially sentencing Anna to death. Castiel wanted to reach out to Dean and assure him that Anna would be okay, even though that would be a lie.
Suddenly everything stopped and Castiel understood everything. Somehow, even though seconds before he had felt sorrow over what would happen to Anna, he suddenly wanted to destroy her.
Castiel watched in horror at the way Dean's eyes and mouth softened as Anna tilted her face to his. He felt his heart drop into his stomach. Suddenly, it were as if his insides were on fire. He saw red and it wasn't just Anna's hair. His right eye twitched and he felt his hands ball into fists.
What is she doing?
WHAT IS SHE DOING?
Anna's mouth moved up to Dean's and she placed her lips on his.
That evil whore! She is the Whore of Babylon!
A series of violent images flooded into Castiel's mind. He imagined walking over to Anna and Dean and grabbing her skinny human body and ripping her spine from her back. He imagined how good it would feel to crush her spinal column in his hands, smashing the bone into a fine, powdery dust. He imagined shoving an angel blade into her chest and the wet noise her human heart would make as it beat one last time. He imagined her grace shattering, brilliant white light streaming from her eyes, her ugly, evil, wide eyes. He imagined torturing her before stabbing her to death with an angel blade, picking out her eyeballs with tweezers and squishing them like grapes, then plucking the feathers from her wings one by one.
When we get back to Heaven, I am going to make that bitch pay!
He stared at Dean, who looked so melancholic, so broken, and suddenly the violent images were replaced by a kinder set. He imagined returning to earth after Anna's death and taking the hunter into his arms, comforting him, making the pain of betraying Anna, all of the pain from Hell disappear. He imagined kissing his forehead, then his lips and then taking all of his pain and replacing it with light and joy.
Anna pulled away and Dean rested his forehead on hers. They looked into each other's eyes for a moment, silently exchanging something that Castiel couldn't fathom. Castiel was suddenly desperate to look into Dean's eyes, to understand what it was he felt for Anna.
What did she do to him? What did he do to her? What is that look he is giving her?
Castiel imagined holding Dean like he was currently holding Anna. He imagined looking into his eyes and knowing absolutely everything he was thinking and feeling. This image gave Castiel momentary, but absolute joy. His heart swelled at the thought of having that connection with Dean. He ached for it.
Anna pulled away from Dean, giving him one last sorrowful look that made Castiel rage again.
I am going to fucking kill her.
Anna came toward him and Castiel felt his vessel's pulse quicken. The utter hatred and anger he felt toward her right now was profound. He had fought in many wars, killed many demons, and watched angel, demon, and humanity do cruel, evil things. He had been betrayed, tortured, and now he was fighting against Satan himself. Despite all this, Castiel had never felt such hatred toward any being before. He had never felt such bitterness and pain. He had never… felt emotion. He still didn't quite understand it. Even the concept of emotion was alien to him.
But am I feeling such emotion now?
Castiel stared at Dean, who was giving him a look that made him feel like he was the size of an insect. Dean was betrayed and angry, and he could sense that. He felt Dean's wrath. All of this wrath was directed at Castiel. Feeling Dean's anger made Castiel feel like dying, made him feel like ripping out his own grace.
Then it occurred to him. He had heard of envy and he had just experienced it for the first time. He did not like envy. His passion for Dean was making this strange, unpleasant, and violent human emotion spark alive inside of him. He would kill for Dean. He would die for Dean. He might even kill Anna because of Dean. Castiel wouldn't say it out loud for a very long time, but seeing Anna's eyes as grapes to be squished and fantasizing about crushing her spinal column is what made him realize he was in love with Dean.