If there were (will be?) truly "good" time travelers, why not prevent the nuking of Japan? Or any of the other countless massacres and genocides throughout hist

If there were (will be?) truly "good" time travelers, why not prevent the nuking of Japan? Or any of the other countless massacres and genocides throughout history and still going on to this very day? Or you know, help everyone to resolve all conflicts without violence? Prevent the mass production of weapons in the first place? In the end this is just a variant of the "If there's a God why does it let bad shit happen" question. It's more likely that there are eldritch Lovecraftian entities feeding upon sentient misery.