I haven't looked through this entire thread but I see a good argument for Walt being ISTJ opposed to INTJ
"I have spent my whole life scared, frightened of things that could happen, might happen, might not happen. 50 years I spent like that."
That is so extremely ISTJ over INTJ and ISTJ over any type. Preparing for danger and being very cautious of the future.
His entire life us to age 50 indicates ISTJ. After that and throughout the show does he seem to be much less conventional and more like an INTJ than an ISTJ. Yes, he does seem much less of an SJ. But his entire behavior on the show could still be thought of relating to that very ISTJ quote. Being scared that he will die soon and trying to prepare for the risk and fallout of this seemingly imminent event.
Even if it deemed that his behavior on the show is entirely INTJ which i doubt, does 50 years or 2.5 years when someone is in a continuous crises situation, truly show an indvidual's personality? I go with ISTJ and the vast majority of his life personally.
Nah, I thought about this too, but the reason he seemed like an ISTJ in the beginning is simply because, well, he's a middle-aged dad, that's a go-to ISTJ stereotype. Not to mention he seems to have been in a rather routine-oriented life for quite a while in the pilot, but yeah, he's not comfortable in it, not one measly bit. The reason being, he's not really an Si-Dom.
Walter White is an INTJ who has had an unfullfilled Ni, until he had a cause on which he could fully unleash it. He considers himself above the life of a common family man, he feels a failure when comparing himself to his former science colleagues, he is always looking for a higher meaning, and he gets more and more ambitious the more he lets his Ni loose, starting to act a lot more natural when compared to his mediocre life before he started making meth.
His fear of future could also be attributed to Dom-Ni/Inf-Se, he's so bent on trying to see the future he completely struggles living in the moment. However, once he learns of his condition of cancer, , he spents an awful lot of time gripping on his inferior Se function (For the record, those grips usually happen in stressful situations, and Walter's entire life was gradually ruining itself ever since getting cancer), and because of this, he lets himself be rather impulsive to live life to its fullest (Initiating sex with his wife more often, buying expensive cars, and even engaging in violence to blow off steam).
And that is the scary thing with unhealthy INTJ's, and that's why Walt is the quintessential realistic example of one. The thing being that they could be anyone around you, really. People think of INTJ's as genius super-villains or even bad-ass heroes and indestructible beings like Batman or Gandalf, but in real life, the INTJ, healthy or unhealthy, may very well be your next-door neighbour living an ordinary life and feeling low-key miserable about it. That's what's frightening about Heisenberg. He could be anyone.
Just to drive my point home, here's a quote from Walt's final confession to Skyler: " I did it for me. I liked it (Ni- Fi loop). I was good at it (Aux-Te).And I was, really... I was alive (Inf-Se)."