The Ottoman ruler most responsible for the decline and eventual disunion of the Ottoman Empire is most likely Abdul Hamid II who was Sultan 1909. He oversaw a period of rapid modernization that directly led to political instability. This instability, in conjunction with the Ottoman’s poor showing in the First World War, is what contributed most to their final break up.
Hamid wanted to be known as the man responsible for modernizing the aging empire, so he enacted a series of reforms to help the Empire catch up to Europe. He started a central records system, built railroads, systematized bureaucratic salaries, and even founded the countries first modern law school.
However, all this came at a price. Hamid used his power to curtail freedom of the press. He also spent so much time developing the countries infrastructure that he neglected to attend to important military matters, which would have helped his country during World War I. As a result, the Ottoman’s were defeated several times and suffered heavy losses. This along with serious questions about Hamid’s circumventing of the constitution led to a coup. His counter-coup resulted in the mass-murder of thousands of Armenian and Christian citizens, further destabilizing his rule.