Reminds me of this article. Now, Hemmens is being a little polemical there and I'm not sure the morality of the books is well-developed enough to support his reading, but there's some interesting convergence anyway.
My take on it is that our judgments of people are ultimately about predicting how well their future behavior will align with our (moral and/or personal) interests. This is determined by their core motivations and assumptions, but our access to that deep structure is often patchy and speculative; if we don't have a reliable enough read on it, about all we can do is fall back on nearest-neighbor matching against their past actions. It's not a binary actions-or-motivations dichotomy, in other words: the more confident about someone's motivations we are, the higher we weight those motivations relative to their actual deeds.
Harry's just found out two important things about Quirrell: that his deep structure isn't as accessible as he thought it was, and that he's willing to use death spells in questionable circumstances. That might not be enough to completely destroy his confidence in Quirrell (and indeed I think Harry's overreacting a bit given the evidence), but it's definitely enough reason to take a step back and reevaluate.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: