Diagoras fought in World War I, and it was then that he decided to only take care of himself. He developed an ambition to run New York City, whatever it took.
In 1930, he secretly served the Cult of Skaro, and ran the work to fit the orders of Dalek Caan, who acted as the Cult's connection to the outside world.
The Cult thought Diagoras was the most Dalek-like human they had encountered, as he had rare ambition and thought like a Dalek. Dalek Sec viewed Diagoras as their best option for the Final Experiment, so Caan took him to meet their leader.
Deep within the Cult's subterranean lair, Dalek Sec revealed to Diagoras that he planned to merge with him. Daleks Jast and Thay protested, but Sec convinced them that polluting the Daleks' purity was necessary, as the Daleks must survive. Sec opened his casing and Diagoras was pushed into his Dalek-mutant flesh. Sec fully encased his body in him, and the casing was closed. When the Final Experiment was complete, the casing opened and Sec stepped out, transformed with Diagoras into the first human-Dalek. (TV: Daleks in Manhattan)