Pacifism is a rough road to follow. It takes far more than just refusing to kill and expecting the universe to comply and roll over and everything to turn out all right. It takes immense will to not take the easy way out, kindness to see the hidden good in everybody, diligence and effort to bring it out into the light, intelligence to find the third option in the arch-villain's standard sadistic choice - and power, power enough to subdue the enemy without dealing real harm, power enough to keep your cool at all situations because you know you can handle it if it goes south, power that you can choose to hold back at your own free will. If you lack the power, you're not a pacifist: you're just a victim.
Edward's pacifism has always been a core trait of his, but it's not really come up in the story in any serious manner, outside of a couple mentions, reluctance to handle a firearm, and of course from having never taken a life so far. Now that it is tested, he is found wanting. He's not nearly strong or canny enough to win the fight while being burdened by his principles. He loses, he suffers, and he pays a heavy price. But he pays the price willingly, does not compromise his tenets even in the face of suffering and toil, and learns from it - takes another step towards maturity. And makes a couple new friends in doing so, which never hurts.
There's also a flashback sequence, which informs us of the very earliest backstory of the setting, how its arch-villain and greatest champion of good both came to be, all the while doing a bit more foreshadowing for the future, demonstrating just how nightmarish it can look when an entire country's worth of souls are sapped out and devoured. It shows and tells just what it needs to, without overstaying its welcome. I also like the little framing device on how it begins and ends with a different person.
Finally, this is the first time I noticed just how Riza got the secret code message through to Roy: the words, sure, but also the secret tap to let him know a code was coming in the first place. Very clever.