I think he's referring to this:
Yeah.
I also remember something about "two sides of the same coin" and that the hellfire/soulfire distinction is more about the user than the source. Like Fallen use hellfire because the Fallen are evil and the angels use soulfire because they're good. If one used the other, it would be because they are no longer what they were. As Yuillegan points out, this was from Jim instead of Bob, so I was wrong about my source on that part.
My best guess is that both hellfire and soulfire are both powered by souls. It's mostly about intent of the user as to whether hellfire or soulfire comes out.
I don't recall if Dresden ever use Hellfire on a Denarian but I wondered if it would have garnered as strong as reaction from them (and other similarly divine beings) as Soulfire? I mean, they almost take it personally that he uses it. We haven't seen Dresden try and use Hellfire while hateful or trying cause horror and harm (attacks on Outsiders don't count I think). I wonder what would happen if he did? Would he convert it himself?
I think all this is evidence of my guess. For example, they way Namshiel reacts to Dresden's use of soulfire seemed to me as if Namshiel still used soulfire. His statements about it were present, not past, tense.
[1]I more meant that Harry's continued exposure to darkness is making him more resistant to influence and temptation than he otherwise might have been, as he possibly has inner darkness.
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[2]Yes she did, but her actions are rather more volatile than Mab's. Never saw Mab throw a meat cleaver at someone for being rude.
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[3]Does anyone else think that Justin's abuse went beyond the merely violent? Obviously it was psychological too, but something Jim said about how Twelve Months is about Harry dealing with a lot of repressed trauma...but yeah Harry had a bad hand regardless.
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[4]I get what you're saying but Bob also says that Soulfire is how Angels do all their "stuff". I don't see how Fallen could operate if they were too different. Jim sort of answered this in a Q&A once, he said that Angels have access to Soulfire and Hellfire but which one they use sort-of determines what kind of Angel they are (Fallen or just normal Angel).
1) We know Harry is being prepared for what comes, so maybe the powers that be are working to expose/not protect/protect just enough Harry to/from temptation to build him up to where he needs to be?
2) Well, not a cleaver specifically, but she's certainly given Harry an attitude adjustment or two ... or a lot more.
https://genius.com/Hank-williams-jr-attitude-adjustment-lyrics3) I don't think Justin abused Harry sexually if that's what you're getting at. I do think Harry was abused sexually by the Red Court Vampires in GP.
4) The way angels work in traditional Christian theology, angels can't make choices in the same way as humans because they exist outside of time. Harry is a human who exists inside of time, so he could go back and forth from hellfire to soulfire if it's just about choice.