A good manga is one where intense scenarios are drawn to instill realism—that some of the characters shown are going to die. This makes each moment shown in manga more precious. If the author keeps pulling this bullshit, allowing MC to save whoever he wants, the suspense wears off. I, as an audience, will no longer feel the tense. I just know every side character MC meets will be safe and sound—saved by MC with no risk what so ever. At that point, why bother making horror genre manga?