I think oda will give arguments that the ancient kingdom was something more grey than we thought. Maybe the first 20 kings when fought ancient kingdom didnt have bad intentions (except for Imu)
The Ancient Kingdom flew too close to the sun. They built wonderful things, but, that technology was also dangerous.
Franky's arc explains this: Franky built his ships to defend/help people, but, they were taken and used as weapons.
Franky's ships being taken and used for evil explains the entirety of the Ancient Kingdom's failures.
The Ancient Kingdom built wonderful technology, but, didn't think hard enough about how it could be used to destroy.
We know that the Ancient Kingdom made the Ancient Machines (I refuse to call them weapons). Pluton was built in Water 7, and Water 7 is CLEARLY anti-government.
I think the Ancient Machines caused a great, accidental tragedy, and the WG then seized them to try to stop this from happening again. Both sides were wrong, I believe.