Fawkes refused to go into the purifier. He wasn't quite sure why, but he felt it wasn't his job. The Lone Wanderer went in and typed in the code instead. Fawkes witnessed the Lone Wanderer crying to the ground in pain, the radiation killing them from the inside out. The pain they had gone through was unreal already, and their death was an honorable but horrible one. Fawkes only truly had one friend in this world, and they had sacrificed their own life for the good of humanity. But Fawkes didn't feel human anymore, and he certainly didn't look human.
He couldn't place why he didn't go in the purifier instead. He wouldn't have been hurt at all, but instead he lives with the regret that he let the Lone Wanderer do it. Fawkes suffered from the constant internal battle between the visceral part of him and the cerebral part of him. There was no inner balance that stopped the fighting. He found himself slipping away each day. He was overwhelmed with sadness as he watched the Lone Wanderer die, but the humans had come to like him. He was allowed to live with them, to walk among them, to help them. But he could never help himself.
The headaches got worse and the extreme pain got worse. A pain just like the pain he felt when he had been turned into a monster. Fawkes refused to believe he was a monster, but his appearance was a constant reminder that he looked like one. His head burned and his muscles ached. Voices told him to do horrible things, to be with the other mutants. Schizophrenia. The doctor had told him it was normal for a mutant, but it didn't feel normal.
In a world full of radiation, mutants, and humans, Fawkes felt absolutely alone. He knew eventually he'd become one of them. But he had to fight it as long as he could. The symptoms kept getting worse, though. He had once fallen asleep, then awoke standing over a pile of dead bodies with a bullet in his shoulder. He didn't remember anything, but he knew he did it. His primal instincts pulled at his intelligence that made him unlike the other mutants. Fawkes kept blacking out, blankly staring, while he did the horrible things he did not wish to do.
He began letting go, letting it take over, letting the monster in him arise. He was alone in the wasteland, screaming in pain. He felt as if the two sides of his brain were pulling each other apart. His head was on fire. A horrifying stabbing pain rushed through his body, poison flaming threw his veins. Fawkes had never really became a full Super Mutant, not mentally, but enough radiation would trigger the monster in him. Daggers banged on the inside of his eyes, while he sat screaming and staring into nothing. No one could help him, no one could coax him out of this this time.
He lost all of his thoughts. He felt his memories slowly slipping away. All of the emotions he felt turned into anger. The remaining bit of consciousness he had left turned into the bad voices. The voices telling him he was a monster that had to do monstrous things. Fawkes had fought so hard to stay together, to not become one of them, but he had lost the fight. He was a monster.