Yeah, it says there's no way to counter the Killing Curse in the Harry Potter universe. What's stopping another universe (like, say, the one Rand is from) from having a counter to it? If Rand has a shield that explicitly stops magic from getting in, then it's probably not getting in.
Then, couldn't the opposite be true? That Rand's shields stop magic from his universe, but perhaps spells from the HP world work/act differently in some way that could get through his shields?
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You beat yourself up with your past. Don't blame yourself, blame the world. Blame God. Blame me.