This is just a short gathering of thoughts on what Cass may have done after the events of the episode The End. It's dark. It's violent. It's desperate. It's suicidal. You've been warned.
Castiel knew it had been a trap. He had looked Dean in the eyes when the order was given. He knew, Dean knew, and the past Dean, he had also known. Castiel was silently thankful that the Dean from his time had knocked out past Dean. It was bad enough to see the hardened pain in his Dean's eyes, let alone the past Dean. Past Dean couldn't possibly begin to comprehend the shit storm they had been through the past five years. The pain, the guilt, the late nights of fighting, crying, and drinking. That's how Dean had become obsessed with finding the Colt. That's how as Castiel became more human, he had turned to drugs and sex as a crutch to release these emotions he couldn't quite grasp.
So Cass had followed the orders of his fearless leader, his best friend, the only one left alive besides Chuck that knew what Castiel had once been, an all powerful angel of the lord. He would do his best, knowing that if Dean succeeded, there was a chance of reclaiming the Earth, and if Dean failed, they were all as good as dead anyway. The ambush had been quick on that second story. Looking at the reactions from the rest of the team, Castiel suspected that they honestly didn't realize it had been a trap. A pang of guilt struck him as he fired his weapon, before he backed into a corner of a room, hiding behind an office desk. Blood poured from his left shoulder, the hilt of a knife still sticking out. Blood crusted on the outside of his right eye, his black hair matted with crimson from a head wound. He had watched the shock fill Risa's eyes as she was shot in the back, falling forward in slow motion, crashing to the ground. The demon walking up beside her, aiming his pistol, firing another shot at the back of her head. Blood splattered, brain matter staining the floor surrounding her.
He needed a minute. To regroup his thoughts. He needed to make it to Dean. No doubt past Dean would have caught up with him. Maybe Castiel could talk to Zachariah, keep this reality from ever happening. He heard the gunfire echo further away from his position. The other members of his group must have retreated. Castiel slowly stood, glancing out the window. He saw Lucifer, standing there in his pristine white suit. So angelic in this Hell hole of a world. He cautiously approached the doorway, finding no demons in the vicinity. He walked over to Risa's body. He pulled the handgun from her hand, tucking it into the waist of his jeans. He caressed her cheek, and closed her glazed eyes. In another life, another time, she would have been a teacher, a mother, a wife. Dying in an abandoned office in the aftermath of a war between Heaven and Hell was not what she deserved.
Castiel stood and felt slightly woozy. He looked at the knife sticking out of his shoulder. The blood had stopped dripping, instead slightly oozing around the hilt. He knew the blade had been serrated, no sense in pulling it out now until he had a way to stop the blood from flowing again. He couldn't risk passing out from blood loss, not until he found Dean. He made his way to a staircase, hit boots clattering on the gray steps as he hurried down the two flights. Making it to an exit, he pushed the glass door open. He briefly remarked in his head that the glass was still intact. Not a splatter of blood on it until his hands had touched it, smudging blood and dirt across the surface. Ahead of him, he could see Dean and Lucifer, facing off. Bending over slightly, he needed to breathe. He was getting lightheaded. A deep gasping breath and he had started running towards the scene, desperate to help Dean.
A body appeared beside Dean, and in a blink, he was gone. It was past Dean that was standing. Zachariah had taken him. Castiel's eyes fell to Lucifer, Sam's hazel eyes glazed over with Hellfire. Then Cass saw him. The Dean from his time, laying on the ground. The Colt inches from him his hand, his green eyes staring into darkness. Castiel was too late. Dean had tried, and he had failed. Castiel's rifle fell from his hands, metal clanking quietly on the soft grass below. He shuffled forward slowly, his blue eyes filled with moisture as he continued to stare at Dean's lifeless body. This was it. Their only shot, and it was over. Earth was gone. The angels had abandoned it, the Croatoan virus had taken out three-quarters of the human race, and those left were quickly running out of supplies. Hell had won. Lucifer had his paradise.
Castiel had stopped moving forward, squeezing his eyes, the tears burning down his face, creating cleaner streaks of skin across his dirt and blood splattered face, getting lost in the scruff of his beard. Lucifer had started forward, waving off his demons who had guns aimed at Castiel. Lucifer reached out a hand to Castiel's chin, raising his head slightly. Castiel opened his tear stained eyes, only to see Lucifer's face, smiling in satisfaction. Pity filled Castiel's mind. Poor Sam, stuck in his body, watching this, his body had killed his brother, destroyed the world. There was no way to save him. Castiel could only hope that Lucifer had actually killed Sam. That was the only reprieve from this world, death. Castiel only had one option he could control now. His stomach churned. He could feel the bile burn in his throat. He continued to look into Lucifer's eyes. A quiet exchange. Lucifer would give Castiel a chance in this paradise he created. They were brothers after all. Out of all of the angels in Heaven, Castiel was the only one that had stayed through everything.
Lucifer's eyes flinched as a gunshot echoed, ringing his ears. He wiped the side of his face, smearing blood across his cheek and hand, his white suit splattered with raindrops of blood and tissue. He looked out to the open void in front of him, then glanced down. At his feet lay Castiel, Risa's handgun still clutched in his right hand, the side of his head blown open. His blue eyes reflecting the cloudy sky, once Heaven. His black hair matted with blood. The green grass surrounding him, slowly turning to shades of red and brown.
It may have not been the outcome they had wanted but Team Free Will had made it to The End.