If Castiel was HYDRA, he was doing a very good job at deviating from all the behaviors James associated with such an agent. If he wasn't, then... Then he needed to find a way to answer the angel truthfully. He was desperate enough to try to believe that this was an angel of God. He was desperate enough to confess his troubles to a stranger. James hadn't realized he could still hope for anything for himself. It surprised him - but= the momentary surprise didn't stop him from pressing ahead.
"Things were done to me. To my brain. To my mind. I.. Sometimes, memories that they tried to erase come back up again, and I can't control... I.... I'm a danger to others around me. I don't want to be. My head always hurts, and hurts worse when I remember things. I had a friend. He still remembers me, but I can't remember a lot about him, and I hurt him a lot. I don't mean to. I can't... can't help it... There's a girl. I'm afraid I'll accidentally hurt her when I lose control to the memories bleeding up. I'm not a good person. I've ended so many lives because they ordered me to. The one who says we're friends, he also says it's not my fault, but sometimes I remember the killing. I could have stopped. I could have stopped myself, I could've found a way - I'm sure I could have."
There was no reason an angel would ever try to help someone like him - a murderer, a souless assassin. But still, he hoped.