I'll agree that his rush to the combat is almost certainly an attempt to keep Harry alive.

The timing doesn't make sense for this interpretation. We know Quirrell starts moving when he notices Harry's intent to kill. But, Harry takes out the troll (and hence any threat to his life) several seconds later, and Quirrell notices this. However, he doesn't seem to stop moving then; several seconds is hardly enough time to blow through much of Hogwarts. Instead, he seems to stop when Harry's killing intent stops. This suggests, to me, that Quirrell was worried about this intent -- perhaps that it wouldn't stop when the troll was dead.