According to the U.S. Code Title 18 Section 1114, the murder of a federal officer or employee becomes a capital punishment-eligible offense when committed "while such officer or employee is engaged in or on account of the performance of official duties."
While some reports put Judge John M. Roll at the scene of Saturday morning's horror by coincidence, the federal criminal complaint alleges that Judge Roll had been notified of the event in advance by Rep. Gabrielle Giffords' staff and was attending to discuss "issues related to the volume of federal cases in the District of Arizona."
The carnage of January 8 was more than a senseless attack on individuals by an unbalanced paranoid, it was an act of brutal violence against the institutions of civil government and all the people. We don't know yet whether prosecutors will seek the death penalty for the accused, but it's not going to trouble me if they do.
Incidentally, the U.S. Constitution clearly authorizes capital punishment. While I'm sympathetic to evolving standards of due process, "deprived of life" still means the same as it did in 1791.