Cas slowly walked over to Sam, knife held up to his neck, and he slowly started to carve through the skin as Sam yelled in pain. Dean shouted at Cas to take him instead, to just let Sam live a normal life, but Cas ignored him. Between Sam's screams of pain, he told Dean it would be okay, everything would be alright.
“Sam, you should’ve died a long time ago. I’m just doing you a favour.” Cas said, with a chuckle.
Sam kept shouting reassurance, "Dean it's going to be okay! You'll get out of this, don't wor--" Cas twisted his hand, snapping Sam’s neck and causing him to fall to the ground.
"SAM!" Dean shouted, but he knew it was too late. Cas looked at Sam’s dead body and then up at Dean, and then laughed. He smiled the same way he smiled when the Leviathans had taken over his body. He had become a monster, worse than any other angel.
Cas vanished and reappeared back where he was originally, in the middle of the path, and he slowly wiped his knife on his coat while smiling. Cas let Dean off the wall, and Dean immediately ran over to Sam, shouting and crying as he tried to wake him up, even though it was pointless.
"Sammy, I'm here. It's okay. Sam, Sam listen to me. Sam! Sam wake up dammit! I need you!" But Sam was long gone. Dean got up and started walking towards Cas, who laughed and prepared his knife. Dean charged towards him, pulling out an angel blade that he had taken from one of the other angels Castiel had killed with him before. Cas wasn’t expecting Dean to have the knife, and was taken by surprise. Dean pushed the knife through Cas’ chest, and he fell to his knees.
“Dean…” Cas whispered, in one final breath, before falling to the ground.