Rereading the passage in Battle Ground after Murphy dies and Harry goes after Rudolph, and something stood out to me. Harry's is of course incredibly angry, irrationally angry (by which I mean he is bypassing his normal rational state for his emotional reaction). He talks about the purity of that hate and how it gets disrupted by something he can't quite explain when Sanya arrives to stop him. It's further interrupted when Butters arrives to stop him. When he tries to destroy Rudolph with lightning, Fidelacchius stops it. It's interesting how the sound of Fidelacchius is described during this encounter as an "angry chorus". When Harry attempts to pass through the sword thinking he will be immune (being purely physical and thinking himself not truly bad), he is burned badly and there is a smell of brimstone. The burn not only doesn't heal well, but Winter's mantle cannot numb it. Then afterwards he feels all his pain but also is made rational again, and realises there likely would have been extended consequences for his actions including on those he cares about most.
This book talks a lot about love and hate. About logic being the opposite. The Eye of Balor is a weapon that uses hate to destroy everything it touches. Mab's logic appears to save her from it, if only just. Harry's love of his child stops him from murdering Rudolph, and the love of his friends helps him realise that and try to stop him. Ethniu's hate is what caused her to attack the world. The Chicagoan's love of there city and their families causes them to fight for it. The themes are clear.
Way back in when we first learn about Denarians, we learnt that it's part of the nature of the Coins to be in circulation. A Denarian also tells Dresden "we are corruption, we are a part of you". This is a hint to the nature of angels and fallen, and why they are so interested in the souls of mortals. I suspect the interest is because of the nature of the soul and it's relationship to the fundamental reality. Jim has openly discussed that choices in the Dresden Files create/spin-off universes and this creates a spectrum of universes that the angels and fallen fight over. Perhaps the souls can do this because they are tied to the foundational energy of the universe - love, and by definition, hate. Which means souls contain both creative and destructive power/interest and the angels and fallen wish to tip a soul more one side than another, and therefore have more universes in their favor.
What does this have to do with Battle Ground? Well as discussed above that book is very much about love and hate, amongst other things. These are called two sides of the same coin by Mab, as it were. Love is fundamental and essential in creation and hate is essential in destruction. This is how the Eye of Balor can destroy anything it touches (providing someone hasn't prepared accordingly *Mab*) including a being like Uriel. The Eye is described as having "hate of the original vintage". This is interesting because what I believe Butcher is talking about is not merely hate as a fundamental part of universal power, but perhaps also what Lucifer tapped into. Whether or not it came from Outside is unknown. But I do believe hate is what makes Hellfire so strong, and love is what makes Soulfire so "real".
So I believe that Battle Ground was actually not merely about Ethniu or Outsiders or even the Black Council, per se. I think we got to see another microcosm of the Great Battle, but it wasn't just over Chicago but particularly over someone in it - Harry. I believe that Sanya and Butters didn't merely show up to counter Ethniu and her forces. We have learned that the Knights single most important purpose is to save souls, particularly those influenced by the Fallen. And while I think Ethniu was considered a grave threat, there were longer term moves going on. Drakul's presence hints at that to a point. But I think when Harry was about to kill Rudolph he was heading for the darkest timeline. That Harry would have been far worse for his universe, perhaps multiple universes and this was another pivotal moment in his life. Is it not amazing of all places that the Knights ran after Harry? Or that there was not some other catastrophe they had to prevent? I would say considering the purpose of the Knights of the Cross, it means quite a bit that two of them were there to stop Harry going dark side.
I also wonder if Harry particularly is susceptible to influence of the Fallen, in a way. Obviously anyone trying to manipulate him directly tends to fail as Harry is fairly obstinate. But depending on what Starborns have to do with Lucifer (and we know of two connections: 666 years and that Jim mentioned it specifically recently), perhaps that same power that allows him to resist Outsiders is also a double edged Sword. Jim often says being a Starborn is really unlucky. People often regard Harry as a monster if they know that he is Starborn. My guess is that Harry is somehow more connected to those fundamental forces, even more so than a normal Wizard, and therefore when he goes dark side it's really, mega bad. Maybe that's why he was burned by Fidelachius. Maybe that's why the hate felt so pure, and why it annoyed him so strangely that he was disturbed from his attempted murder. Perhaps, as others have guessed it's why Drakul is so different from other Starborn (so far). It should also mean Harry is just as susceptible to the good things (read: love) though I would think, but I don't feel the evidence is just as strong.