I think Walt's reasons for going into the business at the beginning were sympathetic, he didn't want to leave his family with a huge amount of debt.
Yeah, all that went away the the moment he went into business with Fring.
It went away a lot earlier than that, pretty much in the first couple of episodes of the show. After all, there were people willing to pay for Walt's treatment. He wouldn't have had to turn to Meth cooking at all but he had already made up his mind. It was just an excuse to finally get the reward and recognition he felt he was owed but never received.
However, I'm also one of those who remained sympathetic to him for very long - way too long, in fact. I did root for him basically until the thing with Brock. To me, the moral ambiguity and the fact that we're made to root for or sympathise with the bad guy is one of the best aspects of the show. I like these ethical rollercoaster rides.