See Five Nigerian Leaders Who Died While They Were Still In Power.

1.       Tafawa Balewa: Nigeria’s first Prime Minister ruled between 1960-1966. He was the first Nigerian head of government to die in office. Balewa, born in 1912 was murdered in 1966 as a result of a military coup spearheaded by Major Kaduna Nzeogwu. He was assassinated on January 15, 1966. That was the first military coup to be recorded in the history of Nigeria.           
2.        General Murtala Mohammed who became head of state in July 1975, was assassinated on February 13, 1976 , following an abortive...
military coup that transpired on February 13, 1976, he was murdered alongside his aide-de-camp, Lieutenant Akintunde Akinsehinwa. The coup was spearheaded by Lt. Col Buka Suka Dimka. Murtala Mohammed died at a young age of 37.           
3.    Umar Musa Yar’Adua died on 5 May 2010 at Aso Rock Presidential Villa. However, before his death, there were speculations that he had been unwell even before becoming president and was imposed on Nigerians by President Olusegun Obasanjo. On 23 November 2009, Yar’adua left for Saudi Arabia due to a medical challenge attributed to pericarditis, a disease condition where there is inflammation of the membrane covering the heart and when he returned to Nigeria on 24 February 2010, people said he was still on life support while some said he was recovering but he never made public appearances until his death was officially announced.
           
4.       Major General Aguiyi-Ironsi. He was assassinated on 29 July 1966, while still head of state as a result of the problem which occurred when Tafawa Balewa was ruling and the fact that Nigerians had started loosing confidence in his government. Irosi was said to have hijacked power and became the first ever head of state to reap the effects of a military coup in Nigeria but his administration which took over power 16 January 1966 was overthrown when he was killed seven months later by a cohort of Northern military officers who ganged up against him. The circumstances leading to Ironsi’s death still remain a subject of much controversy in Nigeria. His body and that of Fajuyi were later discovered in a nearby forest.
         
5.     General Sani Abacha’s administration (from 1993 to 1998) was alleged to have carried out widespread human-rights abuses. The exact cause of his death is not well understood but sources claim he died of food poisoning at the hands of two Indian prostitutes who poisoned him with an apple. Abacha died in June 1998 while at the presidential Villa in Abuja and was buried on the same day, according to Muslim tradition, without an autopsy.