I'm going to try and rewatch the series and last night I just watched the grey matter episode from season 1 and while I don't agree with Walt's actions....I understand. His old partner had Eric Clapton at his birthday party, Walt doesn't even have HDTV. Having to tell everyone else at the party your life went to crap and you're a high school teacher while your partner is loaded and chilling with Eric Clapton because he piggybacked on your breakthrough. That's enough to make any man go crazy.
Also at the foreshadowing, Hank's card analogy during the intervention and season 3 with Walt & Skylar making up the card counting excuse for why Walt suddenly came into all this money.