A Ukrainian court sentenced a former security official to life in prison on Tuesday for the death of Georgy Gongadze, a journalist whose mysterious death in 2000 provoked an international outcry and helped set off protests against the president at the time, Leonid D. Kuchma. The former security official, Gen. Olexey Pukach, who once headed a surveillance department for Ukraine’s Interior Ministry, testified that he had not intended to kill Gongadze, but strangled him with a belt accidentally in the course of an interrogation. Gongadze went missing in September 2000 and his body was found two months later, beheaded, in a forest 75 miles from Kiev, the capital. He had infuriated the president with muckraking publications in Ukrainskaya Pravda, an Internet newspaper he had founded.