The leader of a small splinter group of Muslim rebels died Tuesday of natural causes, the head of the Philippines’ military said, removing one barrier to peace in the south of the mainly Catholic country. The leader, Ameril Umbra Kato, organized the Bangsamoro Islamic Freedom Fighters to battle for an independent Islamic state. Gen. Gregorio Catapang, armed forces chief of staff, said the rebel leader had a heart attack in his hide-out in Maguindanao Province. Mr. Kato was previously a field commander of the country’s largest Muslim rebel group, the Moro Islamic Liberation Front.