Roy serves as a deuteragonist to Ed and Al, being the other main viewpoint in the series. Roy is part of the military, part of the system - which is evil because the homunculi have been secretly pulling the strings and using the military state as a way to cause wars in order to sow as much despair as possible in order to obtain a Philosopherās stone.
Roy is a man who intends to do good, as do all of his team who are very loyal to him. Unfortunately, his choices, to join the military to make a difference, led him to Ishval where he participated in a genocide.
Itās interesting to think how Ishval was presented to people such as Roy - early in the series we hear Edās beliefs about the Ishval massacre just being a riot which the Ishvalan people started and the Military responded. What Ed doesnāt realise is that he was being fed military propaganda. Roy wouldāve been receiving lies as well to motivate his actions.Ā
Itās perhaps a bit telling that itās the murder of a pair of Amestrian doctors that were so clearly not monstrous enemies that really shook Roy. He has nightmares about the Ishvalans heās killed as well but he was able to justify all his actions up to that point as just being in service to a greater cause and making a positive difference by quelling the rebellion. This act against two Amestrians helps him to realise that the whole thing was wrong and he couldnāt justify his actions any longer.
So he wants to make changes but and he believes that comes from remaining in the military to change it from within. But ultimately, he canāt make real change - the Ishvalans continue to be mistreated by the military and heās forced to continue quelling their attempts at resistance. The situation in Liore really highlights how little has changed despite Royās great efforts.
Learning that the Fuhrer is a homunculus only confirms everything he feared - about how little power he has in his position despite the rank and respect. So he tries something new.
Armstrong, Fuery, Havoc, his whole team are happy to help him stage a coup while his most trusted companion Riza joins him to take on the Fuhrer directly. They are all loyal soldiers who believe in doing the right thing and they believe in Roy. Thatās the one thing he has truly gained. And it makes all the difference.