Like the 3rd or 4th episode of the series he rejects an offer by Grey Matter to be a consultant where he'd be paid a bunch of money and have access to the top doctors in the world. Anybody really looking to just help their family takes that deal, not decide to stick with drug dealing.Walt initially figured he was just in it to help his family but obviously in the end he was in it for the power trip. He was ruthless too, but I think he entered into the drug business not expecting to live long and not knowing where he would end up.
I think even the creator of the show says that's when Walt breaks bad.