Atty. Gen. Dick Thornburgh pledged to seek the death penalty for the killer of an unarmed federal drug agent killed during an investigation of cocaine dealing on Staten Island. The vow came as a civic group, Citizens Outraged at Police Being Shot, posted a $10,000 reward for information leading to the arrest and conviction of the killer. Hundreds of federal and local law enforcement officers pressed a nationwide manhunt for paroled murderer Constabile Farace, 28, the prime suspect in the killing of Drug Enforcement Administration Agent Everett Hatcher, 46, shot four times at close range Tuesday night. Farace could become the first person tried under the federal death penalty law targeting drug kingpins and defendants convicted of drug-related killings of law enforcement officials.