What happened was an accident.
A sheer twist of fate that had in turn stolen even more from them than just the semi transmuted life of their dead mother and the physical being of Alphonse Elric. It had also stolen two limbs, created more trauma than neccessary, and in the end...changed Edward Elric from a natrually born male- to a young girl with very precious few memories of Alphonse Elric.
And that was perhaps part of what the young boy found so cruel.
They had known from the beginning that trying to bring their mother back from the dead was a taboo that would cost them. But they had never imagined that it would cost them so damn much. In the span of mere moments the two siblings had had everything they had ever known ripped away from them.
The warmth and love of their mother... Again.
Alphonse his body- before his brother's intervention.
And Ed...an arm and a leg coupled with the second price of a physical transformation from one gender to another, along with the loss of the first several years of his-er-her, Alphonse meant her-life. And along with that second price, the knowledge that his brother would eventually pay a heavier toll.
A transmutation circle had been seared into the skin of his brother's new body. Directly over the stomach where once upon a time male insides resided. And with it came an ominous warning that any with or without the ability to use alchemy could use it to ensure that their seed would take root.
Whatever that meant.