"Cas totally dug his own grave on this one, and I don't actually have very much sympathy for him. I have sympathy insofar as it's clear to me that Castiel now regrets his actions and realizes his mistakes, but that doesn't LESSEN his mistakes or actions nor does the simple act of regret atone for it all."
This above is exactly how I feel about Cas too-I don't think Dean acted too harshly.

Thinking back the secrets he was keeping is probably why Cas was trying so hard to convince Dean that putting Sam's soul back would only hurt Sam (irrationally ignoring the torment Sam's soul, which IS Sam, was going through). Though to be fair no one else really mentioned that either-just that soulless Sam wasn't Sam and would do bad things.