Do you think it could be because he's Michael's vessle? Like, I remember Cas thinking he'd be able to hear his real voice when he was first introduced.
I don't think that it's "only angels can kill angels" but more of "only angels have the weapons to kill other angels." After all, Gabriel made a comment about how he'd never let Kali have his sword because she would be able to kill him with it. Plus in The Song Remains the Same, Cas made mention that Anna wasn't holding a knife that could kill angels. I don't think it has anything to do with the angels themselves. Just the knives/swords/whatever the fuck they are.
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