Sam wondered why he was alive, and why the gates hadn't been shut. He murdered Kevin, plain and simple. Granted, he was possessed by an angel named Gadreel, but still. He couldn't decide who he was more angry at: Dean for letting him get possessed, or himself for not completing the trials.
Castiel understood. After all, he was human.
"Being human didn't just change my view of food," He said, "It changed my view of you."
Sam hopped off the table. The angel later informed him that there wasn't enough grace for the spell. As if Sam didn't already feel like a failure.
"Don't worry, Sam," Castiel assured him, "We'll find Gadreel another way."
Sam nodded and sat down. Castiel barely looked at him.
"What is it, Cas?"
"Have you forgiven me, Sam?"
"For what?" The hunter was confused.
"What I have done to you. The harm I have caused due to breaking your wall."
Sam laughed, "Cas, I forgot about that."
Castiel squinted.
"You've forgotten Hell?"
"Well, no. Not all of it. But…you took my insanity away."
"I have avoided you somewhat, out of fear that you couldn't love me anymore."
"Couldn't lo…How could I NOT love you, Cas? I know that you've hurt me, but I've forgiven you. I forgive you, even Dean for letting me get possessed. I just…can't forgive myself."
"Why not?"
"I've done so many…horrible things…"
"How many of those THINGS have been out of evil, Sam? Tell me."
"Well…"
"That's right," Castiel said, "You are a good person, Sam. You can't save everyone, but you want to, and it hurts when you don't."
Sam hugged Castiel.