John W. Loftus asks the question in this post at his blog.
When terrible things happen, God is hidden in a way that make him seem non-existent.
So, why pray for someone to be saved, or to travel safe, etc, etc?
Furthermore, I think it's obvious that the more power someone has to avert a tragedy then the more of a moral responsibility he has to interfere with it. If I encountered a gang of thugs beating up some kid I cannot be expected to physically stop them. But a superman who came upon them doing so should. So also with God, the ultimate superman.
The fact of the matter is that we do not have as much free will as commonly supposed, if we have any at all, as I wrote about here. Since we don't have that much free will anyway, there should be no objection to interfering when someone wants to commit a heinous crime like James Holmes did.
If an omniscient God somehow needs to judge us then he can judge our thoughts and intentions alone. He should therefore stop people like James Holmes dead in his tracks before he acts on his thoughts.