I feel like prez's deal in the overall story will be introducing Satsuki to the dilemma of whether it's right to save absolutely everyone and to the consequences of her messing with fate so often. He himself is not in the right with his mindset of letting mere bullies die, but it will build up to worse cases that could make Satsuki question herself, such as the already foreshadowed Akira's father who might try to kill Akira and her mother. As for messing with fate, prez noted that potential deaths around school have increased considerably since Satsuki started to prevent them, which will either be because of Komachi like someone else mentioned above, or because of the universe trying to course-correct. Both cases would lead to the argument that it's better to let some people die so that more can live, especially if/when we get to the event teased at the start of chapter 1 and people do die because of Satsuki's activities.