Because of Imu though....Imu is the moral corruption
IMU alone isn't.
It's likely that 20 kingdoms united to form some sort of democratic council with no one to sit on the throne as Supreme ruler. IMU somehow hijacked this to gain absolute power. Lily leaving them and playing role in scattering PGs indicates that WG derailed from their original purpose.
So, WG runs to maintain order and stability but IMU seeks absolute order and they became totalitarian. It's willful or forceful obedience to maintain stability.
With IMU on throne, entire generations of CDs got morally corrupted.
If I have to guess then the reason WG even United in the first place is the Davy Jones rule was Absolute chaos - Law of the Jungle, a complete deviation from what WG seeks.
Ideologically, if I have to sum up,
IMU and WG have bad means to achieve good end / goal. End justifies the means.
The Goal (Good End in their eyes): Global order, absolute stability, and preventing war or chaos.
The Means (Bad): Genocide (Buster Calls, destroying Lulusia), human slavery, erasing history, and total suppression.
Davy jones perhaps had bad means with bad end / goal. Means justifies the end or Might is right.
See how BB operates and what he seeks and you will understand. It's absolute anarchy.
Luffy will have good means to have good end / goal. Both Means and Ends are important.
Davy was bigger evil imo.