[nsfw]My take is obviously that it's largely the latter (though whether it can be blamed on jealousy or innate character traits is another matter), and that's why I find the series so fascinating...what was for the first couple seasons -- both for viewers and most of the characters -- a story about "fundamentally decent family man does terrible things out of desperation" morphed over time into "fundamentally evil quasi-protagonist manipulates everyone in sight" in ways that, IMO, cannot be squared simply with the increased stakes/desperation.
Take the famous
"I am the one who knocks" scene. Skyler tries to justify Walt's behaviour...he's a good man, he was under pressure, he got in over his head. Walt, on the other hand, clearly revels in what he's become; he's not a victim, he's mother****ing Heisenberg. [/nsfw]