Dean wouldn't know about Sam's caution. Aside from the brief conversation about the Leviathan, Dean hasn't talked to his brother. He should, he just hasn't. He doesn't want to know what Sam thinks of him still sticking with Cas.
"We're as prepared as we can be." He takes a deep breath. "I'm gonna kill this son of a bitch, Cas."
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"We're as prepared as we can be." He takes a deep breath. "I'm gonna kill this son of a bitch, Cas."