I think you kind of entirely missed the point of Delita in the story of FFT. Ivalice as it was could only be ruled by someone as unscrupulous and manipulative as Deltia. Even if Ramza killed Delita the person who succeed him would be just as bad. For Ramza to rise to a position where he was in a position to change anything he would have had to become an existence just like Delita, who you so despise.