Why does Mustang address Hawkeye informally after dispatching Envy?
When Mustang comments to Hawkeye that he made her do through something dreadful again, he uses "kimi." 

Kimi is the subjective pronoun of "you" and is used informally, typically when talking to close friends, younger siblings, but not to parents, older relatives, superiors, etc.

Normally, they have a typical superior-subordinate relationship (noted by Hawkeye herself that Mustang doesn't call her "Riza" even when they're alone) where they talk to or address one another formally, but here Mustang is being frank with Hawkeye as a close friend rather than a subordinate.