Per Xotl, I've got permission to post this errata:
PDF Pg 5, story "Hard Justice", second column, Paragraph 2:
"By the power vested in me by the Curia and His Holiness, I excommunicate you and condemn you to eternal Purgatory."
Problem: No Catholic would ever say that sentence -- first off, ecclesiatical powers do not come from the Curia (Their closest analogue would perhaps be the Cabinet here in the US), so no priest or other religious would ever claim authority in the name of the Curia. [edit in for clarification -- Catholics believe that such authority comes from Christ, who gave the keys to "loose and bind to St. Peter, and that priests act in persona Christi at certain times through the sacrament of Holy Orders.] More problematic is "and condemn you to eternal Purgatory." Catholic dogma (which is unchangeable) declares that Purgatory is a place where souls are cleansed of any remaining venial sin or lingering temporal effects of already-forgiven mortal sin in preparation to entering Heaven. It is not a place to be condemned to and by definition it is not, and cannot be, eternal, nor is it synonymous with Hell. Nor can any Catholic actually condemn one to Hell... though given the character's state of mind, he might have forgotten that in the heat of the moment.
[edit in for further thoughts -- Further, though certain members of the clergy can delcare excommunications for specific acts (and it takes a lot of doing to get a declared excommunication imposed -- usually it's for very public heresy or attempting to consecrate priests or bishops without Vatican approval) following a tribunal investigation, according to the Code of Canon Law (which is changeable, to a certain degree), an attack on the Pope is cause for automatic " latae sententiae" excommunication the moment the act is committed.
And if that member of the Greenhaven Gestapo wasn't Catholic to begin with (the story seems to be silent one way or the other), pronouncing excommunication would be pointless anyway -- it's an ecclesiastical penalty. A Catholic priest imposing an excommunication on a non-Catholic would be like ComStar declaring an interdict on a planet without an HPG.
At any rate, excommunication does not expel one from the church. Even if excommunicated, a Catholic still is obligated to go to Mass on Sunday and Holy Days, but they must not receive Communion until the penalty is lifted. It also bars the one under it from receiving the other six sacraments -- a member of the clergy, usually a bishop or the Pope, with authority to lift the excommunication is necessary for the penitent to celebrate the sacrament of confession. It's also not synonymous with "damned to Hell."]
Suggested fix: Perhaps something like "'You excommunicated yourself when you helped to murder Pope Clement. May God have mercy on your soul.' As he pulled the trigger, he whispered ...'for I cannot.'"