What a clusterfuck. Why go forward with the timeline where three innocent people are dead, one key hero is crippled, and trust between everyone has broken down? The MC's powers mean that he could solve this effortlessly.
The MC should have either killed himself, or gotten one of the others to do so, so that he could go back in time and try to stop this. Even without using any detective work, he could just kill each of them in turn until he kills whoever the Demon King's champion is, rewinding each time he kills the wrong person, so that all the innocent people will be alive.