Castiel looked down at Samandriel. He knew he had to kill him, but he didn't know why. He felt like he had to kill him because he was turning on him. But, that was lining up with the evidence before his eyes. He took his angel blade and stabbed his young brother.
"Cas!" Dean yelled, and he and Sam ran up. "What the hell?"
"I had to. He was going to kill me. It was self-defense." As he said the words, he knew they were lies, but at the same time thought they were the truth.
He made his excuses and left. Again, without knowing why, he felt the Winchesters were a danger to him, and he to them.