That seems to suggest that Harry has entirely given up on the idea of judging people by what they do... at least, when it comes to Quirrell.
Quirrel is a schemer and what he does or rather wants you to think he does is not what he really did. Let's take the dementor for an example. What Quirrel did was bring a powerful creature within reach of Harry with the effect of almost killing him. But also with the effect of teaching Harry the true Patronus. How would you judge Quirrel according to this deed? With a more straight forward character you could assume he really wanted to teach the children to defend themselves which probably is good. And Harry realized that this straight forward reasoning doesn't apply to Quirrel.
In this instance Quirrel probably realized something of the problem with good and powerful wizards could not cast Patroni and thought that Harry's absolutely odd way of thinking of things might be able to solve it. It would make sense that he should do this quickly as his plans to turn Harry into a Dark Lord might get in the way of this.
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: