That's the title of a recent article by psychiatrist Ronald Pies at Psychiatric Times (free login). As Dr. Pies states:
There are some crimes so heinous, they may simply be beyond the human capacity to forgive: genocide, murder, and rape come quickly to mind. Gangdev (following Mahoney et al, 2005) defines forgiveness as the "...releasing or foregoing of bitterness and vengeance by a victim toward the perpetrator of an offence, while acknowledging the seriousness of the wrong."5 (italics added) Thus, none of us--other than the surviving victims--is in a position to forgive the Tucson shooter for the wounding or killing of innocent bystanders.