Complex sentence: Hideo finally realizes that Tekko is dead and kills her via decapitation with a kitchen knife, then ceremoniously cleans the apartment, worrying about the police, but then leaves a notice under Tekko's note that says he is turning himself in.
Split into simpler sentences: Hideo finally realizes that Tekko is dead and kills her via decapitation with a kitchen knife, then ceremoniously cleans the apartment, worrying about the police. He then leaves a notice under Tekko's prior note that says he is turning himself in.