True, it's a bad decision, but she thinks that the family is desperate for money. Her fear of instability is what pushes her into amoral acts. Walt on the other hand had an alternative to making meth right from the get go - the money from his former partner at Grey Matter. It was his ego that drove his decision. He couldn't bear the thought of surviving because of someone else's generosity; it had to be his own talent and work that was responsible for his well-being.