Glorfindel's starting threat is a joke!
As the other reviewer said, his stats warrant 12 threat. Even if we completely ignore his willpower due to the negative associated with it, that would still make Glorfindel a hero with 9 stat total, so that's what his threat should be. When looking at Glorfindel this way (i.e. pretending he has 0 willpower), his negative card text can be expressed as a positive perk, as in "Raise your threat by 1 to add 3 willpower to Glorfindel until the end of the round", further justifying the more balanced 9 starting threat value.
Furthermore, his low threat is exactly the opposite of what we would expect of him thematically. Threat in this game represents the enemy attention we bring to ourselves. That's why the more powerful the heroes, the more starting threat they have. This fits perfectly with the theme of the books, where a high-profile heroes such as Aragorn, Gandalf and others that Sauron deemed to be... well, a threat, drew his attention away while a "nobody" brought the One Ring to be destroyed to Mount Doom. Importantly, the reason Glorfindel did not join the fellowship was that he was so powerful that he would draw all the unwanted attention. So who the heck thought it a good idea to make him the second lowest printed starting threat in the game (and the first at the time of his release)?!
It makes no sense either mechanically nor thematically for this hero to have such a low starting threat. I have Glorfindel in my pet deck myself, but am seriously considering adding a house rule errata to make his starting threat 9.