A few loose ends he'd have to take care of here. He couldn't go around killing someone to break the day's loop until he was sure no conclusive evidence was left behind. The first murder clearly showed the danger of having left clues in the crime scene. Secondly, the pattern of murders was already too clear and closely strung. It's not random enough, narrowing suspects could be more easily done if the police assumed the first murder connects with the rest (since the first victim would be the odd one out, not being a student of the same high school as the rest of the victims). And then there's also the question of whether Katagiri lied to the bullies, which would make her a witness and a dangerous piece to let continuing the day.
Here's to hoping kid stays careful and methodical.