There are many theories as to who ordered the assassination of Ngo Dinh Diem but the reason why is more explainable: his dictatorship of South Vietnam, persecution of buddhists, and outright corruption did him in.

His land reform program put the land in the hands of the privileged and rents skyrocketed. Buddhist officers were denied promotions and discriminated against for their religion (Diem was a Roman Catholic). US aid went into the pockets of his supporters instead of building a strong nation to compete with the Communist North in the upcoming elections.

It was obvious that Diem would not be a puppet to the US, elected by his people, or run a fair democratic state as had been hoped. Whether the Communists, his own government, or the US ordered the assassination is still up to debate.