User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-31.53.226.53-20120607165416/@comment-5110811-20120611144109

Here's what you don't seem to get:

Angels don't have souls, but they are not evil. They somehow possess human-like emotions. That's how God made them. they are good natured soldiers built to follow. Their "good" actions and motivations are beyond and outside the human psyche/morality. In their own minds they see the "bigger picture." Which is why they have no qualms of killing innocents for the purpose of the greater good in the long run.

Ever since Cas busted Dean out from hell, he was intrigued by the human concept of free will and their moral compass. So he wanted to "understand" it. To learn it and spread it to his fellow angels. Like he tried to do with Rachel and his small army. But as pointed out by Dean and Hester, Angels don't have the "equipment" to care or empathize on a human level. And when they try to do, it breaks them. That's why Cas went all insane.

Leviathans, are a completely different story. Like angels, they too are soulless. But they are pure darkness geared to total evil. No compassion, No empathy, No emotions. Pure Chaos. They are by definition - Pure Monsters. The very first original apex monster.

And Sam's torments are specifically meant for Sam only. Lucifer didn't break Cas. When Cas absorbed them, he said Lucifer was nothing more than just an aftertaste. When Lucifer's illusion left his psyche, It was the overwhelming guilt of killing thousands of humans and his fellow angels that really did him in. He did these horrible atrocities for the sake of "the greater good" in his warped twisted mind. That "broke his ass."