Very contrived trial. If he had access to even the most average lawyer, this one short admission of guilt alone wouldn't stand in any proper trial. I've heard a lot about the flawed japanese court system, but I don't think it is that flawed.
I can understand that the author didn't want to spend too much time on the prologue. Cheap intro for a barely believable case of "killed with strong resentment". The investigator is so cartoonishly evil he breaks any semblance of credibility.
Otherwise, the art so far is below average and the start of the competition seems stupid. The whole thing should be started for everybody at the same time. Here, a system such as "Battle in five seconds", with at least a first phase where the competitors are on equal starting grounds, would make much sense. This reincarnation cup looks more like "Darwin game", that starts with a setting closer to mobile games than a proper competition.
Also, is the reincarnation cup an actual public event? Or is the translation completely off? The MC seems to know something about it even before the masked girl went into details.
(Edit: scratch that last part. It's just a jumbled mess because of usual MTL mistakes and random switches between the narrator's exposition and the masked character's explanations.)
I'll read the next chapter, but this first chapter feels off on so many levels.