So, Imu has been waiting for hundred of years for Luffy alias Joy Boy.
He has an ability which is called Domino Reversi or something like that which allows him to tempt and control anyone in the place they recognize as their home.
He's a bit like the devil and he signs contracts symbollically.
The Swords of the Celestial Dragons ancestors are planted before Imu's throne as a contract, recognizing that they are recognizing Marygeoise as their home which means that Imu Can at any time change the Celestial Dragons into demons...
But there is more, every king joining the WG (World Gouvernment) must plant his Sword too, which means that they also recognize Marygeoise as their home and this, from Marygeoise, Imu Can change them and their descendants into demons slaughtering at his orders.
Luffy is immune because of his fruits but the vast majority of the people in this world aren't immune just like Rocks D Xebek, his home was God's Valley, home to the Davey clan which he belongs to.
When he was fighting along with his crew, Roger and his crew and Garp with Dragon against Imu, Imu used the Domino Reversi to control him and make him destroy his own clan, even trying to kill his own son : the future Marshall D Teach.
Teach was saved and then Garp and Roger allied themselves to bring down Rocks.
The Domino Reversi might be the reason Roger was executed in his own island, in case he escaped.
But there is a frightening point I realized : the admirals must come to Marygeoise to pledge allegiance to the WG but it might be a mean to make them involontairily recognize Marygeoise as their home, signing an involontary contract with the Devil.
It might be the true reason Garp always refused to come to Marygeoise !!!
In fact, if this is truly the case, the Navy could have never rebelled, the admirals always have a Sword of Damocles above their heads...
So in the end, Luffy will only be able to fight with his alliés belonging to non WG affilated country like the Giants, the mermens, the Wano Swordmen, the "Angels", the citizens of Alabasta, the citizens of the country freed by Dragon and the citizens of the very poor country, I think we could even expect Doflamingo transforming into a demon because of his origines.
So yeah, Imu's plan is well thought. Every bad action of the WG which might just look like extrem case of corruption is in fact a part of Imu's conspiration.
The citizens of non member country aren't treated like humans because they don't bring anything to Imu and also to motivate more country to join.
So yeah, the World is in a deep shit because the Marines can't do anything against the WG and because the WG was never established to bring peace to the world but to allow Imu to obtained more demonic soldiers, to control the word's fate in his hands.
The WG is nothing more than a vast conspiration...