Where's the line?

In chapter two of The Problem of Pain C.S. Lewis talks about God interfering with the creation. He explains how sometimes God does disturb the free will and natural laws to perform miracles. In class it was discussed that if God sometimes intervenes with a person, why not stop terrible atrocities from happening. Hitler’s taking over of Germany and the Holocaust was mentioned. I see a bit of a problem with this thinking. It makes me wonder about natural disasters like hurricane Katrina. Should God have stopped the hurricane from wiping out New Orleans? What about serial killers or school shootings? Should God intervene at those times as well? At what level of atrocity should God intervene and stop it from happening? The question I would ask is where would God draw the line?