I don't think HWWB was involved, simply because of the gas station incident when Harry first encountered him and blew him up. I also doubt that Harry even knew at that point in time how to summon an Outsider, and was totally freaked by his battle with HWWB. Lea may have had something to do with Justin's defeat, though Jim has said that all she gave Harry was confidence.
Like I said, Justin may have been over confident, got careless because he thought he knew what Harry could do, and that was all that Harry needed. If I remember correctly Justin's death was due to a lot of KABOOM from Harry causing the house to burn down and him with it. Remember the KABOOM was so big that until Summer Knight Harry thought he had killed Elaine along with Justin.
HWWB needn't have been *intentionally* involved.
Just the act of Harry unbinding the Outsider-summoning (Harry only could manage it because he was a Starborn, & it was an Outsider) might have had all sorts of nasty consequences for Justin...
If there was even a fraction of a second where HWWB's only-partially-bound power (as it is being dismissed back through the Outer Gates) might have backwashed onto Justin, he could still have been in
very bad shape when Harry went back ...
Entirely hypothetical theorycrafting. But WoJ is that Justin is
dead-dead; and, as noted: a 16-year-old half trained wizard (even with Harry's native strength) is unlikely to have taken down:
- a senior warden
- with enough combat-chops to have been in on the final showdown with Kemmler
- plus the added power-ups from Black Magic (well known to be a shortcut to extra power).
That kind of combat wizard just doesn't let their guard down enough for
anyone to "sucker punch" them, no matter how confident they are that the foe isn't a threat.
So it's worth looking for some additional reason for Justin to be
dead-dead. HWWB is a good reason, right to hand.