God's Will?

I awoke feeling appalled and horrified at the shooting in the Aurora, CO theater...as did probably every other American. As we sit through the media handwringing over what made the shooter commit such a heinous act......what is possibly a rather intemperate thought crossed my mind.

Just days ago, George Zimmerman proclaimed that his shooting of Trayvon Martin was - "God's will".

My intent is not to pass judgment on a malicious or benevolent God, but rather to understand mankind’s utilization of this explanation. This theme is used often in what I believe are mostly attempts to soothe the stricken or simply fill as a placeholder for a tragedy that bears no understandable motive or reason.

But what are the parameters for using this reasoning? How do [presumably faithful] people measure when an act is "God's will" or when it is the sole crime of man?

Is this theme the genre of personal and individual instances of grief and tragedy, or can it [or is it] applied to events as large as the Aurora theater shootings?