300 seconds
It's natural to be afraid of dying. At least for humans. But it's never easy with the Winchester brothers. Looking at Dean standing calmly in Stull cemetery and arguing with the two most powerful archangels, who have decided to wear the younger Winchesters as their expensive meat suits, Castiel realizes why Dean is not afraid of dying: he is already dead inside.
Castiel understands that this is the end, that there is nothing they can do to fix it. It's too late. Everyone will die: Dean, Bobby, himself included. Michael will kill Lucifer, as there can't be any other way. The blast of the battle will raze the city to the ground, taking the lives of thousands. Maybe it will destroy the whole country. The world. Castiel doesn't know.
Castiel had always hoped for his father's return, that God would not allow the chaos and anarchy the seraph's older siblings caused. He had never wanted the day to come where he saw Dean Winchester dying in front of his eyes. And it's tearing him apart. Helplessness and desperation attack his mind, scratching and clawing at his very being, filling up his cells with their venom.
Castiel does not understand his own actions, but he becomes obsessed with the idea of prolonging Dean's life, even for a few minutes. He would rip out his grace, wings, heart, and soul just to keep Dean safe. One could ask, what about the rest of the world? If he had to choose between saving Dean and the world, what would Castiel choose? Of course Dean Winchester. He had always been the angel's priority. The world could go to hell, commit hara-kiri and spill its guts out, if he couldn't save Dean.
If he had to picture the end of the world, Castiel would choose a different scenario. He and Dean would be sitting on the scorched ground in each other's embrace, sharing a half empty bottle of Whiskey and looking up at the raging skies, pouring rain of fire down on them.
Castiel clenches his fist around the bottle of alcohol with a burning rag stuffed into it. His eyes find Dean's face and the angel gives him a small nod, with a faint smile on his lips. Castiel is sure that it will be the last smile Dean will ever see on his face.
"Hey, assbutt," the angel rasps, drawing Michael's attention. The holy fire engulfs the oldest archangel and with an earsplitting scream, he disappears.
"Assbutt?" he hears Dean ask incredulously, but he ignores it.
"He'll be back, and upset," Castiel shrugs slightly, "but you've got your five minutes."
Castiel understands that he's just signed his own death warrant. And the icy cold voice of the Morning Star removes any remaining doubts.
"Castiel," it's so hard to look at Sam Winchester's face knowing it's not him. "Did you just Molotov my brother with holy fire?" The devil makes a step towards the seraph.
"Uh… No," Castiel denies, taking a few steps back.
"No one dicks with Michael but me!" Lucifer hisses, narrowing his eyes.
Before the fallen archangel snaps his fingers, Castiel has a few seconds to think about the choices he's made, regret things that he has never found time to do, like telling Dean how beautiful he looks when the sun rays play with his freckles in the mornings while he still sleeps, how he loves seeing Dean fixing the Impala with his hands covered in oil and greasy stains on his cheeks and forehead, how he loves to hear Dean's sincere laughter, how he loves Dean Winchester…
The last thought flashing in Castiel's mind is to ask for Dean's forgiveness. "I am sorry, Dean. I am sorry that you have to witness the end of my worthless life. I couldn't allow you to die before I met my own death."
A wise man said: "For what is it to die but to stand naked in the wind and to melt into the sun? And what is it to cease breathing, but to free the breath from its restless tides, that it may rise and expand and seek God unencumbered?"
Castiel doesn't feel physical pain when Lucifer snaps his fingers. His mind is so overloaded with thoughts about Dean that Castiel doesn't have time to feel pain when his body explodes into millions of molecules and atoms which are scattered around the universe, his grace spilled and creating new, flawless, mesmerizing galaxies.
The only pain he feels is from the thought of leaving Dean behind, that he will not be there when the hunter draws his final breath, that Castiel will not be there by Dean's side to squeeze the older Winchester's hand gently and lie shamelessly that everything will be alright.
And Castiel realizes that all of his fears of leaving Dean behind, letting him die without the seraph being there by the hunter's side, crash like sand castles when Castiel finds himself standing on his knees in front of his father and creator on a deserted beach, somewhere along the Pacific Ocean.
Castiel knows that Dean Winchester will not die today, not when God caresses the angel's cheek, smiling fondly at his son and whispering:
"Go to him…"