One of the most defining parts of Roy as a person has to be, without any doubt, his ideals, as I already mentioned. He's easily the most idealistic person out of the whole Fullmetal Alchemist cast. Since Roy's evolution through time comes along with his change of views towards the world, and the adjustment of his beliefs due to his own experiences, I deemed necessary to make a section solely dedicated to this.

Roy: [...] Even so, if I could become one of this country's foundation stones and be able to protect everyone with these hands, I think I'll be happy. That's the reason why I learned alchemy, but... in the end, I wasn't able to be taught Master's secrets. ... Sorry, I ended up speaking of my naive dream.

The above concept serves as the basis of Roy's ideals. A beautiful dream, but nevertheless based in the wrong idea that the world is a place less complex than it really is and that people don't wish for anything else that the other's happiness. Roy would soon find himself in the middle of a corrupt world, one in which evil people exist and a king doesn't wish for the well-being of his people.

Still, he doesn't give up. Even after going through a horrifying experience such as participating first-hand in a massacre, he can't give up. He knows that the possibility of changing things is still within the military, even if through a different path than the one he imagined. And thus, he can't run away from what he already considers his duty. Instead, he comes up with a different approach, a more realistic way of making the world a better place.

Roy: [...] The strength of one person is limited. In that case, I'll protect all I can... even if it's just a few... I'll protect what's important... In turn, the people below me will protect those below them. For a small human, at least that should be possible. [...] If we become unable to talk about our ideals, the evolution of humans will come to an end.

     

He leaves Ishval with that idea in mind, and with the desire and energy to overthrow Bradley and his tyrannical mandate. But he's aware of the fact that he can't do this alone. Hughes gave him the final push and the reassurance that he could make it to the top. He entrusted Hawkeye with the duty of looking over him, not only for the sake of his security, but also so he wouldn't betray himself and step off his path. And finally he made sure to find some comrades who would believe in him and to whom he could trust his life. And they in exchange could do the same.

Roy: I am a powerless human. Because of that, I need your help in order to protect everything. I will protect your lives. And you will protect only whom you can... even if it's only a few, protect those below you. And those below you would also protect those below them. No matter what happens, live, continue to survive greedily. Live, and let's change this country together.

Roy used his experience, and the pain he and the people around him had to endure, to polish his ideals till they became something plausible. They were reborn into a new kind of hope for the future, one in which Roy and the people that support him believe fervently, one they're ready to defend, no matter what it takes.

But also, a dream which isn't completely perfect either. With the possibility of setting things straight, comes responsibility for their previous actions. Roy has seen beyond the point where he reaches his goal and it's not so bright for himself. Claiming power over Amestris as Fuhrer and continue with an unfair dictatorship would be a half-assed and selfish win, something which doesn't suit Roy at all. The right thing to do, then, would be to install democracy and abolish the National Alchemist institution. Something which would deprive Roy of the protection his uniform provides.

It's likely and logical that Roy (along with other people who served in Ishval) would then be judged for his actions. A hero in times of war becomes a mass murderer in times of peace. Things could go really bad for Roy given the worst case scenario indeed. But he knows this and still keeps going forward. Because he also knows his own future and happiness are a small price to pay compared to the greater good of the country. We still ignore the outcome of this, but I doubt Roy would let the people under him to go down too. After all, "protect those below you" could also be applied here.