In order to make this claim, you should point out that Diem had ruled South Vietnam for almost 10 years before he was killed. After the death of Diem, the country went through a string of rulers, with coups happening multiple times per year. This can be seen as a turning point because it made it much more difficult for the South Vietnamese government to seem legitimate to its people and for it to attract support. As this became harder and harder, it became more and more necessary for the US to take control of the war. This led (one can argue) to the Gulf of Tonkin Resolution and the Americanization of the war. In this sense, the assassination of Diem was a real turning point.