The reason why they killed 75000 Jews was because in the non walled cities, they did not pick up on the signals the king was sending that he want the decree nullified. In the walled cities they did so when they got the second letter that the Jews can fight back, it was understood that the King does not want the Jews Killed and “vish Lo omad Bifnayhem”. Thats going on just the walled cities.
In the unwalled cities, the people got a letter that said “kill the jews” (letter 1) – which was not opened until the 13th of adar. They then got a second letter, Esters letter that the “Jews should kill”. So there was a war from the Die hard anti-semites and the Jews won because they were backed by the army.