Very committed to the "guilty until proven otherwise" premise, I think it's from japanese ethics.
How there will be made hard proof in this situation? That's the hook.
Personally, I'd already be speculating their relationship but it should be apparent that he's a copycat. The bodies of dozens of identical persons should be proof enough, if not only the change in modus operandi.
Crap, but that'd be procedural drama! Not Akumetsu! Very bad judgement on my part, very sorry.