He is preventing it, just not instantly as you and I would prefer. He gives people seventy or eighty years to act out their desires, and some see the light—but most never do. If He gave people a much shorter time, that would reduce the horrors, but then He would be accused of being unfair for giving people inadequate time for repentance.
Meanwhile, He works constantly and in many ways to limit and reduce humans’ assaults on each other.
If it happens, by definition it has not been prevented.
You can't start from a logical fallacy and get anywhere other than illogical conclusion.