In the flashbacks, Roy is treated as an extra. I think this is brilliant. He's the lower-rank soldier that the viewer usually doesn't give a second glance to; he is the staple guilt-ridden gun of a higher-up in that scene. But we *know* Mustang now, that is the difference, and suddenly we're hit with the raw cruelty of it. We're forced to sympathize with that 'extra', and realize all the other times we have not sympathized with that 'extra'.
Yet another reason why Full Metal Alchemist is quality.
Just had to say it. ^^; *crawls back into hole*