Disclaimer: I don't own FMA:B. If I did, Mrs. Bradley would have had a happier ending.
11. Anger
Scientifically, anger is a biochemical reaction: an automated response to external negative stimuli. Intellectually, it is merely an emotion, masterable by human conviction.
Practically, however, it is alive.
It lives and breathes in the most primitive corners of the human psyche, white-hot. Burning, blazing, blinding; searing away reason and rational thought until only impulses remain, until even conscious decisions are clouded by red impressions. Blood rushes into your skull, fueling a dizzying haze. Some become intoxicated from the scent, from the energy anger brings to their spirits: some crave, even, the tingling flood of adrenaline. It sinks into the very flesh, toxin-like, before finally staking a vicious claim on the heart. It is a rabid, frothing sensation. A crazed madness in which specifics are erased and all of conscious decision driven away until you're a slave, an animal, bowing down before the will of a higher power, before a manic and more sentient being than yourself. Anger becomes both the god and the altar, maroon-stained stones the foundation of a wicked religion.
Despite its power, however, it passes. Anger, by nature, is a transient beast, and as quickly as it rears its ugly head, it vanishes into a mist of denial. This hot, pounding rage is experienced daily, and, in an instant, forgotten. Casualties are considered accidents, and angry words spoken while mad with bloodlust are forgiven.
No one is held accountable, really, for what they do when they're not themselves.
But for action, there is a reaction. For every extreme, an opposite. Duality of nature is common in science, and, therefore, in both people and their emotions as well. Most forget that anger can take many forms. Yes, it has the capacity to run its course and pass with time, much like a fever. But, if not treated, it morphs rapidly into a fever your body cannot sweat out. It digs its way into the seat of your mind, festering, growing cold, anticipatory. Parasitic, it shuffles into quiet reticence, sighs, and dies, before being reborn as hatred. As a cold flame. Reason sharpens into a point, and contrary to the directionless energy in the previous, there's a premeditated tilt to the cold. You are directly responsible for your actions in every sense. It becomes a singularity of conscious focus and drive. Revenge, too, is born of a cold fire.
The danger of such emotional sores lies not in the primal fight-or-flight response, or in the reckless reactions, but in the icy shell that follows after the firestorm. There's a chill that settles in the bones of the afflicted after the heat has consumed itself, and a void - a black hole of emotionless waste - follows. Hatred is the biggest danger, because hatred makes you the sentient killer, and disguises all else as disposable.
Here is the place at which monsters are made.
Monsters are not human.
Monsters are cold.
"Congratulations, Colonel! You figured it out!" Gracia Hughes melted, revealing a grotesque and wicked interior.
Envy is an ugly beast, Roy thought. But even an ugly beast can be made to suffer.
And, with a shiver running up his spine, Roy knew what it was like to feel cold.
AC: Roy Mustang isn't a monster. He's actually one of the characters who exhibit the most empathy and higher human thought. However, he still came very close to losing himself when Envy decided to tug him along for that emtional rollercoaster. It speaks to the fallibility of human nature. Good thing Hawkeye was there to snap him back.
...and I suppose Ed and Scar helped too.