I feel that Walt is displaying the classic loss of the ability to reason when exposed to meth. He's not using meth AFAWK but he is certainly exposed to the chemicals every time he does a batch.
What I'm shocked about is Jesse... He started out way more bad in season 1 than he ended up in this season. Perhaps his meth use has helped him be more resistant to exposure to the chemicals of the cook.
They wear protective gear sufficient for a commercial operation. The chemicals wont be affecting them. Walt is displaying the classic loss of the ability to reason when exposed to raw power and after finding that killing people is getting easier with practice.
