It was the end of the world. Hell had literally broken loose. After Sam accepted being Lucifer's vessel Dean focused of building a group to stop him. Cas was the first on board. He could exercise demons and heal the wounded, so he was very important in most people's eyes. But the connection with heaven started to sever and Cas grew weaker. He had no idea how to handle a human body, his grace dulled the distracting senses of his human vessel and without it he didn't know how to cope
First it was little things, noticing smells, becoming more sensitive to light, being able to feel the slightest touches. At first Castiel thought it was just earth changing from the end of days; it wasn't until he started to get weak from hunger did he understand, he was becoming human. With his powers fading this wasn't something he could hide for long. Exercising demons went first, luckily they weren't on a scouting mission when he found out; a woman's protective pendent had been damaged.
Dean had made quick work of restraining her, "hurry Cas!" he yelled as the woman hissed and bite at him. With his usual stride Castiel walked over and placed a hand on the woman's forehead, but nothing happened. They only had time to share a shocked look before the woman wiggled her way out of Dean's grasp and try and get a taste of human flesh, but Dean stabbed her with the demon's blade just as she opened her mouth. He pushed her off the blade and stared at her while he wiped the blood off.
When the blade was clean Dean tucked it into his belt and glanced at his friend. Cas was staring at his hand with a less than neutral expression, but anyone other than Dean might not have noticed, "Hey, you ok buddy?" Castiel let his hand drop before replying, "I must be fatigued." Dean new that wasn't the issue, or at least the whole issue, but pressing for information wouldn't help right now, so he refocused on the dead body in front of him, "did she have family in the camp?"
"No," Cas finally looked at his friend, "she came alone." Their stares lingered for a moment, "Good," Dean took the knife off the woman belt, "then there's no one to explain to and a new knife for someone." he tucked the knife by his before heading back toward the main base, "You comin'?" "Give me a minute" Cas glanced at the body then back at Dean who had already turned his back and left. Cas rearranged the woman in a more dignified position, "I only hope heaven is still open for you."