Ehhh, it's not THAT uncommon for older couples to grow some resentment. The whole boomer, "I hate my wife" DOES come from somewhere, and spiteful people that are legally bound to each other will do all sorts of stupid shit before they split.

... Though usually it's from people a little more shitty than Walt. The sort of people who turn into selfish egotists a little easier than getting a cancer diagnosis and a bag of cash. Granted, the show did a great job showing how they were both flawed and both had a touch of ego in their normal lives. Walt's just happened to explode feeding off his resentment of his old business partner, and recent success producing.

In Skyler's defense, she was staying because she was an accessory after the fact, not that she was terribly OK with it. I've definitely seen people make that level of dumb decision before, and about as big of life decisions, too, with far less payout than millions of dollars and a viable laundering scheme!