There are a certain percentage of people in the world who will kill people for no reason, molest children, order executions and market crystal meth to young mums. This has always been the way, and will always continue to be.
Now I'm not belittling it as a tragedy, but we have to accept that some terrible things just happen and cannot be prevented or deterred. They always have, and they will continue to do so.
Sometimes there is no lesson to be learned, and we have to accept that what we've seen is a manifestation of human nature, in the same way that lightning is a manifestation of nature itself. Neither are predictable, preventable and both will happen forever.