Ok. So then. All the earlier antagonism was 100% misplaced and from a place of self hate from Koga. But also kinda dumb and shitty for him to have shot down Shigemoto's ideas about how the area and other circumstances may have had a play into what happened to Alice. Not only cuz we know now in hindsight that Shigemoto was probably right and the other shithead played a role in the incident, but because Shigemoto is literally grieving after his Sister died? Like bro I know you probs also cared about Alice and shit but Shigemoto is literally her brother.
It's a li'l goofy how it's depicted as if Koga was closer to Alice somehow or held a stronger relationship. Like I imagine he loved her and possibly was in grief when she died also but bro literally went about it the worst way by not trying at all to help Shigemoto get closure, then proceeded to lay the blame upon THEM both (even though in his mind he later blames himself) start ostracized Shigemoto and being needlessly Hostile when they reunite. Etc etc. Like overall just a sign of an inflated Ego tbh. Thinking everything is about him. Though glad to see he was motivated by taking care of the magical girls and now works better with Shigemoto and co but yeah overall kinda goofy ass.
Modorikawa truly MVP cuz he came in as a host and helped Shigemoto deal with his grief when Koga was out being a shithead.
I think you need to focus on the first part of your comment: Koga hates himself over Alice's death.
It's pretty clear that he fully blamed himself for what happened to Alice. The pain was still
incredibly raw and he could only look at things from the lens of guilt and self-loathing. At that specific moment, I don't think he could accept any explanation that some outside force might be responsible since it would feel like he was denying his perceived responsibility or trying to find excuses to minimize her death. Koga is very big on responsibility, so saying "It's not MY fault she died!" when he fully believed it
was just felt disrespectful and callous to her memory. Shigemoto bringing up his theories right then while Koga was sinking into his own grief just struck a VERY raw nerve.
I don't think this was intending to imply Koga was closer to Alice, either. The flashback was his, so it's focusing on his feelings rather than Shigemoto's. And like you said, he clearly hates himself for Alice's death. He wasn't in a position to help Shigemoto investigate because he was just too caught up in his own grief to look at anything else. It's the classic "emotions vs logic" binary: Koga fell completely victim to emotions and couldn't look at it logically, while Shigemoto was able to focus more on logic.
It's a bad match-up. I've seen and participated in many heated arguments with that sort of dynamic, where the logic-focused one causes a blow-up from the other person because
why the hell are they able to talk about this stuff so calmly!? Do they feel NOTHING!? There's just something about the other person being so weirdly calm while you're on the verge of a breakdown that
really sets people off. I've been on both sides, and I've seen friendships end because of it—and it was over MUCH smaller things than someone dying.
One final note: you keep talking about how Koga wasn't helping Shigemoto deal with his grief, but I feel like you're minimizing
Koga's own grief in the process. And based on this flashback, no one was helping him. It seems like he broke ties with everyone in his old life to start over, which means he was totally on his own with this self-loathing and self-blame. He channeled it pretty productively into advancing safety in the magical girl industry, but that sort of self-loathing tends to leave people pretty bitter and miserable overall. He lost the idealistic enthusiasm that had spurred him in the first place.