He assumed he had to sell drugs so his wife and kids would be taken care of after he had passed. But his wife had a decent job with a pension and unbeknownst to him was willing to cheat on him. She could have easily remarried after he died. Everyone would have been ''okay''. The shows entire events are redundant.
shareWalt was a loser. He either got shafted or left in a huff from Gray Matter and then took up the lowly job of being a high school teacher thereby squandering his talent. He married Skyler who was a bottom shelf looking out for herself type and unattractive to boot and his son Walt Jr was a gimp and a whiner.
I don't think Walt was paying much attention to what he was doing. He just seemed to be living off the scraps he found on the floor.
So Walt was repressing all of his anger, disappointment and resentment while at the same time pretending to himself that everything was just fine. It was only a matter of time until his inner lobster rebelled and he went over to the Dark Side.
Really I think Walt and Skyler kind of deserved each other though for different reasons.