Kouta's determination has been since Kachidoki Arms was introduced was to prevent the mistake that he made in the beginning when he killed the Inves form of Yuuya and choose the lives of himself and Mai over that of Yuuya. The current situation essentially mirrors the one he was confronted with in the very beginning: facing a personal friend and member of Team Gaim driven out of his mind and bent on killing him, but he is forewarned this time.
How could you compare Yuuya's situation and Micchi's?
- In Yuuya's situation (Kouta vs Yuuya eps. 1), Kouta
did not have any idea about the real identity behind that inves. He fought simply in order to survive. He had psychologically nothing holding him back to fight. He only knew about the identity of that monster much episodes later (eps. 22) thanks to Takatora. Not to mention that Yuuya only attacked Kouta because his mind had completely lost forever thanks to his turning into an inves.
- On the other hand, Micchi's situation (Kouta vs Micchi eps. 38) is completely different than that of Yuuya's, Kouta currently knows the opponent he fights is his friend (Micchi) so of course that fact has held him back to fight. Plus, as far as Micchi's insanity goes, his mind
has not completely lost because he has not transformed into an inves just like Yuuya did. It makes sense that Kouta still thinks that his "talk no hissatsu" will eventually convince Micchi. Even if Micchi did turn into an inves and finally totally lost it, Kouta could still not kill Micchi, if you considered Hase's case (where Kouta were not able to kill him).
However he is so deranged that he belives that hope is nothing but a disease and that it's more heroic to sacrifice billions of people just to save a few. But the reality is that he sold out the human race to the Overlords.
Well, as bad as Micchi currently is, at least he has not completely lost it just like Yuuya or Hase.