Human transmutation was the one thing alchemy advise not doing. It was against all the laws in alchemy; it was the taboo of alchemy.
It didn't matter that every alchemy book warned against it and every teacher warned against doing it too.
Fullmetal had still done it. Him and Alphonse for the love of their mother. Fullmetal was willing to do anything for her.
They figured everything would go back to normal. Al would be happy and he would be too.
He would see his mother smiled again. It was the only thing he wanted.