Voldemort's motives have been a mystery from the start - why would he become a Hogwarts teacher, and what does he want with Harry Potter? He must be working on some kind of plot, but what? It's not a sneak-in-and-kill-people kind of plot, judging by his behavior.
To me, the speech and the conversation with Harry afterwards aren't further mysteries - they provide some of the best clues we've gotten so far about what Quirrelmort is thinking. He sees a conflict between wizards and muggles and is trying to make sure that the wizards win. Coming to Hogwarts to teach battle magic and mentor Harry Potter could fit with that motivation, and ewbrownv does a pretty good job of filling in some more of the details.
In canon, IIRC, LV actually did want to become a Hogwarts teacher.
Regarding personality shifts; is it possible that this is another point of departure, and we're looking at a LV who made fewer Horcruces? Again IIRC, there seemed to be a linear relationship depicted in canon between Tom Riddle's physical and mental decay and the number he created.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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