Al couldn't cry.
"Don't do this," Hohenheim gasped. "Please, Al. You aren't a murderer. You wouldn't kill your father—"
Al cut him off before he could babble anything more. The sword sliced down under the man's jaw, through the gristle and between the bones of his spine without too much effort. Al thought, grimly, that he was getting good at this. It helped that he put extra effort into making sure the sword he'd created out of part of Hohenheim's machinery had a an extremely sharp blade. It also helped that he'd done this a few times now and out of sheer practice had figured out the most efficient way of getting the gory but necessary job done.
As soon as the head was completely severed Al dropped the sword and grabbed the body part, picking it up by it's long shaggy hair with one hand and cupping his other hand under the severed neck. He'd learned the first time he'd cut off the man's head that if he didn't separate it, it would heal closed almost immediately. If however he held the head apart it would take much longer, and more of the man's stolen lives would be burned in trying to put himself back together. The head felt terribly heavy, and though Al used all of his massive strength to hold it still, he found the pull insurmountable. After a few seconds, Al's fingers gave way and the head pulled free and snapped back to the neck as if pulled by a strong magnet. The wound healed seamlessly. Al dropped the hanks of hair left in his hand.
For a couple of seconds Hohenheim remained stunned by his death. Al used the time to reinforce the bindings that held the man's body to the concrete floor. The straps would not be comfortable, Al knew, but it was necessary that they allow no wiggle room whatsoever. If Hohenheim ever got the upper hand again, then—well it was too terrible to even think about.
"Al, you can't kill me," Hohenheim said. "And you shouldn't. I AM your father."
Al clapped his hand and alchemically sharpened the blade of the sword again. "You aren't my father," he said. "You weren't there to raise us, to keep me and Niisan safe. When mom died, you left us alone to fend for ourselves. You let us make TERRIBLE mistakes. You left us to suffer. Five years ago, I owed you half my genetic makeup, but even that's not true anymore, because of you."
Al brought the sword down again before Hohenheim could reply. He was glad he didn't feel exhaustion. In some ways being Armor had it's benefits. Niisan would have been worn out by this gruesome exercise long ago. He could do what needed to be done, and when he was finished, his faulty and fading memory would come in handy as well. This was one memory Al did not plan to hold on to.
One, two, three, four—the head pulled free of Al's fingers and began to heal.
"You may not acknowledge me, but without me you would not exist," said Hohenheim. He seemed colder this incarnation. Al wondered if maybe the ancient alchemist was coming closer to the end of his lives. Al hoped so.
"I do have one question," said Al. "Why did you even bother to have children? If you didn't want to be part of their lives. Why go through all the effort. Don't tell me that Niisan and I were accidents. I know enough of biology and alchemy to know that you couldn't father kids without going through extensive measures. So why did you do it?"
Al held the sword over Hohenheim's neck and waited.
The older man's eyes narrowed. "I worked for centuries to make you and your brother. I needed a woman who with sufficient intelligence and alchemical potential, and yet who was naïve enough to allow me to experiment on her—your mother was a find. The perfect host for my experiment. And beautiful as well. I was lucky. For a while it was entertaining to play the part of husband and father, but I truly had more important things to do, and so that hobby had to come to an end."
Al felt a ghost of anger. Mother had LOVED this man, and he obviously felt no more towards her than he would towards a laboratory rat. Al severed Hohenheim's head again.
When it reattached, Al spoke. "Why did you make us?"
Hohenheim said nothing.
"Yes, I see, why search the world for powerful alchemists when you can make them yourself."
Hohenheim's eyes narrowed. "Is this about your body? I can get it back for you. Just stop trying to kill me."
"If you could bring back my body, why haven't you already?" Then Al shrugged, knowing the answer. "It wasn't a priority. You only need my soul, is that it? As long as I can do alchemy, who cares if I have to walk through life like a freak. As long as Niisan can do alchemy, you didn't care about the pain he has to live with every day. We are truly nothing to you. Ingredients in an experiment. Human sacrifices."
Al couldn't cry.
All those years, all those things he could have had—the taste of food, the smell of flowers, the touch of his skin against another's. He remembered them well enough to miss them keenly. Other kids his age played sports and thought about kissing girls and dreamed of what they would be when they grew up. Al couldn't look forward to marriage or children or old age. He'd never know the mysteries of sex. What father would want to cripple his son so?
"I once wanted to find you," Al said. "Before I realized what kind of man you were. Now I wish I hadn't. Now I wish I was like Niisan and never cared. I know it was stupid of me to fantasize about having a father who would love me and be proud of me. Finding out that my father is a megalomaniac and a mass murderer, who thinks of me and niisan as an experiment and material resources—I didn't need to know that.
"You want to know why I'm killing you? Because if I didn't, Niisan would have to. Even if you can't love him and protect him, I sure can. I'm not murdering you, I'm saving my brother. I'm saving myself. I'm saving my entire country."
Al cut off his head again and held it out. One, two, three, four, five—He held it with all his might. After twenty it seemed like the pull was dying down and it was getting easier. At one hundred, the pull was almost completely extinguished. Al continued to hold the head until suddenly something snapped and the flesh disintegrated under his gloves.
"I'm not a murderer," he said to the collapsing mass. "I'm a hero."
Al couldn't cry.
When Ed and Mustang and all the others found him an hour later, he was sitting next to the machinery. "Did you see your father?" asked Mustang.
Al looked at the dust and shook his head. "No."