It is inevitable that people will call God into question when tragedy strikes. No one is appalled that sinful humans get to breathe air they don't deserve today; but everyone is appalled when God removes His restraining hand and allows evil humans (all of us) to act out the evil already in their hearts. Unless Jesus comes soon, this will not be the last time an evil gunman (or terrorist) acts out his or her wickedness. We must be ready to suffer well and we must be ready to love suffering people well. I think each and every Christian needs to have a "theology of suffering." By that, I simply mean that every Christian should understand the Biblical teaching on suffering.
Thus, here are some of the best resources to help us build Biblical categories for suffering:
1. How Long, O Lord? Reflections on Suffering and Evil by DA Carson
2. Suffering and The Sovereignty of God by Piper and Taylor
3. The Message of Evil and Suffering by Peter Hicks
4. When God Weeps by Joni Eareckson Tada
5. Far as the Curse is Found: Human Suffering and the Hope of the Gospel (audio CDs)