Learning about the science wasn't forbidden. Knowledge was a virtue, after all. It was simply the very act of opening that backdoor into the universe, of reshaping the world with such ease, of making use of that corrupting power, of thinking oneself akin to Ishvala Herself that was against Her laws, and for good reason. Ezra knew it was for good reason, so why was there that temptation to activate the circle, to make use of that heretical energy? He couldn't. He shouldn't.
There was a horrified realization that he'd been building up to this for some time. He'd fooled himself into thinking that he'd only been studying the books, drawing out circles, as an academic exercise, when really he'd been testing the boundaries of what was allowed a little more each time, taking small steps along the path to heresy until it had culminated in this. He couldn't deny the urge within him to see if it would actually work for himself, wrong as it was. Even now, it was possible he could have resisted, could have broken free of the siren call drawing him ever closer, could have scrubbed away the circle with extreme prejudice before he could cross that line, but thoughts of his mother urged him on.
She put on a good face, but knowing her like he did, he knew she'd been pining away for the asshole who'd abandoned her ever since he'd left. And, well, their so-called father had been an alchemist, a heretic, he knew (just one more strike against him). Granted, the man had at least had the good sense not to publicize it, though that hadn't stopped the rumors floating around. Despite his and his brother's uneasiness with the science (and the morbid fascination they'd held for it, nonetheless), their ishama had looked at the little trinkets the bastard had crafted for her, at the crackling energy of his transmutations, with a loving look in her eyes, looked at them as if they were things of beauty. She missed him, Ezra knew that, (even if he couldn't quite grasp why anyone would continue to love a woman who had well and truly abandoned her without even a look back (he hated him, he hated him so much, he'd never deserved their mother and everything, everything was all his fault)) and well, if it was to make her happy, surely- surely that wouldn't be too bad, right? It wasn't like he wanted it for himself, and selflessness was a virtue, right?
Deep down, he knew that they were only flimsy justifications, but nevertheless, it was enough to tamp down his resistance at that critical instant, and suddenly, his hands were on the circle and the raw energy of the firmament was washing over him and he was trying to modulate and shape the flow of power the way he'd read all the finer points of in the books and had never once put into practice, not until this very instant. It was a complex circle, one with all the minute aspects of the transmutation itself drawn into it, so as to leave as little room as possible for user error, but even so, the barely-harnessed power was overwhelming, exhilarating. He'd never felt anything quite like this before (by all rights, he never should have felt anything like this). Before his eyes, the metal twisted, shaped, deformed, forcing itself into the form of a cow.
In the back of his head, he knew that this was a slippery, slippery slope he was going down, but in that moment, he was just too full of sheer awe and exhilaration, the power rush of pulling shape from the firmament and giving form still crackling through his very being. Reality would come crashing down on him eventually, but right now he was just gleeful, eager to show his mom what he'd done. (What had he done?)