S7, E22: Cleo breaks a vial of AIDS-tainted blood, which pierces her skin. Carter gives Chen a good peer review and applies for a job elsewhere. Kovac submits Abby's application to reenter medical school without informing her. A rampaging gunman kills and injures over one dozen children and adults. Greene realizes the killer is Derek Fossen, the father of the little boy Greene had placed in protective custody. The vengeful Fossen killed his son's foster caretakers and shot a cab driver en route to his next destination--Greene's house. But police wound Fossen and bring him to County General. Greene transports Fossen to surgery. Alone with the conscious but restrained mass murderer in an elevator, Greene purposely allows him to go into cardiac arrest and die.