If I’m allowed a bit of speculation
Ikki spent a year on the trail of the red-eye guy. To escape from Ikki because he wasn’t ready, Red-eyes possessed Subaru and is currently dormant inside of him, and the reason why Subaru could not look Ikki in the eye is because he’s unconsciously being influenced by Red-eyes who doesn’t want Ikki to find out that he’s possessing Subaru.
So let me get this straight. This man is using an active volcano as a healing chamber… and you want to fight him.