[michaelochurch]

"Fly" was a brilliant episode, but I didn't take it to be about things that "should never" happen (assertion failure) so much as about Walter's mental decay. The reality is that 100.00% purity is physically impossible to achieve and there are always "contaminants" and people have to balance impurity and risk against the upside of what they're doing (if you're making meth, a fly is not a big deal). In part, I think he was driven by his memory of previous work and a bit of nostalgia. Since he remembers a pristine working environment when he was an esteemed chemist, he's trying to get back to that. In reality, there were imperfections, but he doesn't see them.

In part, it was like Lady MacBeth's "out, damn spot" monologue: a guilty conscience leading to obsessive mysophobia. But it also was the first sign that, even though Walter was at a point where he could just follow orders, coast, and make a lot of money, he wouldn't. Something would piss him off and stop work or worse. His guilty conscience, at first, was about the fly but, later, it was projected onto Jesse, explaining why he took extreme measures (endangering his own life) to protect him.