I don't feel I understand the motivation of Mustang very well, but I do like the support that he receives from Riza, especially. That being said, it feels that his anger would make no difference - killing Envy or not killing Envy is somehow irrelevant in this context. His anger, his revenge, aimed at Envy, didn't feel like it was affecting anyone else. I'm not sure about the moral lesson here, or the logic behind it all.

But it's nice to see the Armstrongs working together.