The MC doesn't think the enemies are roleplaying. He thinks they're terrorists.
The MC isn't dumb enough to headshot someone and still think that they're LARPing when their brains are splattered all over the floor of the school. He thinks that Shadow Garden and Diabolos cult is role-playing.
If you remember the first chapter, the MC was killing bandits. He knew they were bandits and knew he was killing them. But later on, he created the LARP of him and Alpha fighting the cultists, thus turning those bandits into Diabolus cult members who abduct girls who are descendants of Diabolos... which turns out to be true. But he does know that he killed those people.