They were on opposite sides, during the Apocalypse. Crowley was essential in the stopping of it, and now rules Hell. He sees Lucifer as a threat, and Lucifer likely sees him a traitor.
[Which technically he is, but who from their world isn't anyway.]
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Why doesn't Luce like him?
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[Which technically he is, but who from their world isn't anyway.]
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[So it's a matter of when they'll clash, not if.]
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[Amused.]
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[In other words, not able to protect himself. And Castiel is pretty sure Lucifer won't adapt to being mortal very well.]
I would prefer if you attempt not to seriously harm Crowley unless absolutely necessary, however.
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[He promises, reassuringly.
Fights pick him.]
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[And he knows Break will likely try to protect Lucifer if it does come to a fight between the demon and the angel.]
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