I'd like to point out that Hurin's heroic slaughter of trolls is actually somewhat questionable. This bit conflicts with all the available texts we have describing the last stand of the Nirnaeth, and the text from which it was lifted remains unfortunately unpublished. I don't mean to say we can't take Christopher at his word that this piece exists, just that the original is not available for examination. There's a little more detail about this in Underhill's phenomenal article on the Olog-Hai of the First Age, found
here.
I'm also personally of the opinion that, like "true" mythology, there is no reason that these tales cannot exist simultaneously in several forms -- some exaggerated, some more realistic -- told by various loremasters and storytellers. I certainly believe the mighty Hurin was capable of such a noble massacre!