One of the simplest and perhaps most accurate explanations might be that this is a disadvantage that comes with the freedom given with life. As we know from the Bible, there is no harm in Heaven:
Whether the harm comes from the Devil or is simply here as a consequence of the fact that humans were banished from Heaven, what matters is that so many lives are destroyed by it.
So God din’t create evil. But why doesn’t He simply stop it before happening? If He is almighty and He has enough power to do anything, why doesn’t He stop the evil?