I actually rather like the canon Ministry of Magic's current definition of personhood, which is "any creature that has sufficient intelligence to understand the laws of the magical community and to bear part of the responsibility in shaping those laws".
Further, certain intelligent creatures such as centaurs have declined legal personhood status in favor of self-governance.
I'm not so much talking about the legal definitions, as about the basic intuitions that form the framework for the moral reasoning that goes into determining behaviour and then the formal laws and systems that govern them.
It's one of the priors that someone with a non magical upbringing may never consider, that the basic foundation of moral reasoning is different for pureblood wizards.
That other sapient beings have weight as moral actors is pretty basic, and if pure bloods were to instead use a different intuition as the starting point for moral construct...
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.