"That...is something I can relate to," Cas admitted hesitantly. "On both counts. When I left, it was partially because I had no desire to act as someone's weapon and follow orders I had little faith in. If I exist as a weapon, I wish to act on my own accord."
There had been Dean and the recent loss of Sam to consider. He'd also considered that matter, but Dean had only been a part of his final decision. Contrary to popular belief, his entire existence did no revolve around the older Winchester. He only hoped the version of Dean that existed there - when Cas found him - would respect that. Of course, knowing Dean, that was highly unlikely and Cas expected to be called a dick at least once.
As for Piotr's lack of religion, it didn't bother Cas. That also seemed common among humans and there was no changing them. Yet another thing he'd learned from Dean, though it had been one of the more trying lessons. "It may be my humble opinion, but I feel a weapon with free will is far more unpredictable. Dangerous in regard to the opposition."
He opened the door for the other man and followed him inside. "And I've met them all," he said of the archangels. "They're angels best avoided at all costs unless your intent is to contain them."