1 theory I have is that Snape may die in book 7. When Harry found out about him being half blood I think he'll let the other death eaters know and they will hunt him down. Then when he knows he can't be a death eater anymore he may try to join back in the order and the order will kill him. The death eaters probably don't know he's not pure blood since he's a legilimens.
I'm not saying it's a great idea, but it's still an idea, right?
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