First of all, your position what a good starting point for a nuanced conversation would be is fair. I don't know the context from which many people relate to Enron and usually, I insult people like this for fun. But maybe it was hearing him speak or his well selected cases and the challenges I face presenting and selling things when I just want to call a pig a pig - maybe I should have focused on his crimes much more.
Fastow actually claims that he didn't believe he was guilty when he pled guilty. He felt he was playing games and working with loopholes that were legally permissible. It was when his rabbi made it clear to him that following the rules is not sufficient to be a good person if you do not also follow the spirit of the rules that he realized that he was clearly being evil.
Fastow has been in business for a while doing a ton of things related to ethics, teaching at business schools and discussing a lot of what got him into jail and how his Jewish faith gave him clarity into what he thought was wrong. Ethics means many things to many people, but if you are in finance/accounting, the devotion to following reporting rules regardless of the underlying reflected reality becomes a religion of sorts. It's unfortunate that it takes a convicted criminal to ask people the question of whether it is a good thing to use "I cannot find a rule that prevents this" as justification for why you can do what you want to do, but the fact that someone asked the question at a time when lots of people are all about the rules even when the spirit of the rules is being ignored was refreshing.
It's impossible to know what is going on inside someone else's head, but to me Fastow's story seems plainly self-serving. I find it very hard to believe he didn't understand
at the time he would get in very serious trouble if people discovered what he was doing.
Even if we accept his explanation that he thought he was following the letter of the law, there are other rules people are expected to follow. For example, the requirements of your job. He must have been aware he was violating standard responsible accounting practices, which -- illegal or not -- was clearly in negligence of his duties as CFO. That's not about ethics, but simple professional competence.
Maybe the guy has truly had a revelation and changed. But his story just rings incredibly hollow. It's very hard to explain his actions as taken by somebody who thought he was following all the rules but trying to be clever and lacking an ethical framework. He was intentionally stealing tens of millions of dollars from the company's shareholders. The only way he really believed this wasn't in violation of some (legal or professional) rules is if he was astonishingly incompetent.
Would learning about ethics earlier have stopped him from doing what he did? I'm a bit skeptical. He almost certainly would have already understood what he was doing was wrong at the time. He did it because he thought he'd be able to get away with it.