Odd as it seems, I am in this 'LARPing gone wrong' camp as well.
The murderer looks so much like a video game character, walking through the site, opening doors, using tools; nothing makes sense but this makes the most sense to me.
Though I am a member of the targeted camp, the actions and dress of the perpetrator do look, well, cartoonish.
As you state, that could suggest larping gone bad. I am thinking, however, that Larping is usually done as part of an established group. As a result, there would be a drift towards costume sophistication.
For example, perpetrator has full police belt. Maybe even a radio and an airsoft pistol. The attack is then performed not using an improvised hammer, but some kind of more SWATish looking pry bar. The seemingly cobbled together costume could suggest a Larper who is not part of a formal group.
@evilwise also suggested a sub variant of Larping where people follow an amateur action adventure or mystery movie uploaded in segments periodically to TikToc and other social media sites.
Some of the people in the movie are LARPing amateur actors. Others say being interviewed about mysterious goings on are not actors. People watching the unfolding movie can look for for clues etc, then launch their own "investigations" that follow the theme.
These movies may also allow for viewers to send in their own content. If the film host finds it interesting, well done (some such movies are evidently pretty sophisticated) and on theme, they then merge the content into the movie.
The totality could support a possibility where the quasi LARPer is "investigating" the subject of one such unfolding movie. The break into the church to investigate- but dont really have a specific goal, and don't know what to do next. They then make an impulsive attack on a "bad guy" who might be a member of say, the "secret cabal"?