For every Jeffery Daumer, there are hundreds, nay, thousands, of compassionate, warm, normal human beings. Christian theologians have always had a hard time explaining the existence, often the seeming ascendancy of evil in the world, so I won't even try.
When I read a story like this it makes me think how much I love my grandson, how precious a gift he really is. Hug your children, your nephews and nieces, your grandchildren and say a pray for this poor family. A better Christian than me would implore you to say a prayer for the killer as well.
Whatever you do, do not give in the easy seduction of despair.
It is very true that even the greatest evils can be turned into something good by the power of God if a benevolent God truly exists. Unfortunately, I am an atheist and I do not believe in God. If this particular God (or God type) does not exist, then evil is nothing but absurdity. This is something that existentialist philosophers ( like Søren Kierkegaard, Friedrich Nietzsche, and Albert Camus) have tried to come to terms with but to no avail in my opinion. I think about God every moment of every day desperately trying to find a way to believe. Maybe someday I’ll get my wish but I currently have nothing but a fool’s hope and not much of that.