In regards to Mustang’s fight with Lust in Brotherhood, a lot of people don’t really talk about the scene where Riza tries to get out of Al’s arms while screaming for Roy. Most people say it’s because she’s running out to protect Roy.. but I think there’s another interpretation hidden behind that. It could also be because she’s trying to stop him from attacking Lust.
Remember those last lines that Lust said right before Mustang killed her?
His eyes were full of blood lust and revenge for what Lust was about to do to Riza. He wasn’t holding anything back or was willing to give Lust any mercy when killing her. This sounds familiar doesn’t it?
Yep. Mustang’s fight with Envy. And what does Riza do when she sees him going insane with blood lust and revenge?
She stops him. She knows he’s not just killing Envy because he’s one of the bad guys. He’s killing Envy and making sure his death is the worst torture imaginable by burning him to death, just like Lust.
Those aren’t the eyes of someone who is in need of help or is frightened. Roy knows his alchemy is powerful enough to kill and torture Lust, and Riza knows it too.
That’s why she tries to stop him. Just like the fight with Envy, once she sees that he’s going too far, she tries to call to him in order to get him to stop. They made a promise that she would stop him when he began to steer towards the wrong path, and in this moment he is doing exactly that. You can definitely say that the fight with Lust fueled Mustang’s blood lust throughout the rest of the series, shining the most in episodes 53 and 54.