An important thing about Roy Mustang:
Roy’s really good at acting. Roy’s also decided that the general mask he wants to put on around people is “cool, stoic, composed, selfish leader” so they don’t go assuming too many things about his real ideations. Since he’s a good actor, he’s real good at putting on this front.
Off to incinerate Maria Ross? Cooler than ice. Facing confrontation about his role in her murder? Ruthlessly chill. Associating with (well anyone really) when other military personnel might be watching? Thermometer’s reading absolute zero.
But of course the keyword to all this is acting. He’s really talented at acting cool, not to be confused with “He is actually cool”. Because whenever he’s not putting on a performance for someone, his whole personality is pretty well summed up as “high-pitched indignant yelling”
In summation: He’s a loser