Two Plot Problems

[pippin_999]

No: HPFGUIDX 187644

Carol:
 Of course, there are inconsistencies, and, IMO, Lupin's claim that Snape was known for casting Sectumsempra is one of them. If that's the case, why wasn't he expelled from Hogwarts and/or why doesn't the whole WW know that he was a Death Eater?

Pippin:
I've already responded to how it could have happened. But making this an inconsistency  misses JKR's point, IMO. There's a bitter, "I should have known" flavor to "Sectumsempra was always a specialty of Snape's."  Lupin is telling us he should never have believed that someone so savage would remain loyal. Of course that's especially rich considering the speaker.

The larger point of DH, IMO, is that everyone has a savage side, and it's wishful thinking to suppose that anyone, even a hero, can keep it under control completely.

Pippin