I know I talk a big game about how it was Walt’s toxic ideals that destroyed him— how becoming Heisenberg actually ended up being not just ~bad~ in general, but bad for Walt (his family disgusted with him, his empire in the hands of the Nazis who murdered Hank, his meth made and distributed without him) and this demonstrates the show’s thesis of, like: sociopathic power is idealized for men but is actually BAD. But, I don’t talk enough about how the actual mechanism of Walt’s defeat hinges on Jesse. The thing Walt does that ultimately ruins him is his abuse and manipulation of Jesse.
If Jesse hadn’t turned on Walt, Hank never would have arrested him at To’hajilee. Without Jesse’s testimony (enabled by him bearing intimate witness to all of Walt’s crimes), and his idea to go after Walt’s cash (arising from his fundamental understanding of who Walt is), Hank had no case.
If Jesse hadn’t turned on Walt, Hank wouldn’t have died at To’Hajilee. Hank’s death is on Walt, in many senses. He chose to put out the hit on Jesse, and he called Jack and his gang to the site of Walt’s defeat. But, I think the show gives us good reason to believe that if Jesse hadn’t turned on Walt, Walt never would have turned on Jesse. He never would have wanted him dead. Walt felt that his protectiveness of Jesse redeemed him, and it was the final line he wasn’t willing to cross. Until he crossed it.
If Jesse hadn’t turned on Walt, Nazis couldn’t have run Walt’s empire and cooked his meth without him. Not only did Jesse’s work with Hank bring the Nazis to To’Hajilee where they could find Walt’s money, but without Jesse and without Walt’s permission to use him however they pleased, they didn’t have Heisenberg’s blue meth.
Only Jesse could have brought Walt down. Jesse was the only one who knew Walt well enough, who had been with him every step of the way. And, with Jesse’s comparable meth cooking skills, he was also the only one who could replace Walt (however unwillingly). And, in terms of moral decay, Jesse was the only one who could push Walt over that final line (by betraying Jesse), and cause him to commit in a final way to his own evil.
So, Walt’s demise hinges on Jesse turning against him. And this is incredibly fitting for several reasons: it’s fitting for the reason I just gave— Jesse is the one best armed to destroy Walt. And obviously it’s fitting because Jesse is our second protagonist, and is the person who our loyalties shift to as Walt more and more becomes the villain of the show. But it’s also fitting because Jesse is the best judge of Walt’s character. He is the person most intimately involved in Walt’s journey, the person who ends up understanding Walt the best, and the person who Walt hurts the most (I know this last one is debatable, but, come on, Walt really did take everything from him, including Jesse’s relative moral innocence). When Jesse turns on Walt— Jesse, who was repeatedly drawn back in by Walt’s influence and manipulation, who refused to let Gus kill Walt even after Walt belittled and verbally abused him— it can’t be for some small, petty reason. When Jesse finally sees through Walt and realizes he needs to take him down, this is the ultimate condemnation of Walt’s actions.
It wasn’t just one thing Walt did that brought about his demise, one little misstep (although, his little misstep with the book in the bathroom certainly helped). It was everything he did. Because that’s what turned Jesse against him. Not one mistake, but a year of abuses and lies and manipulations, brought into sudden clarity for Jesse when he realized Walt poisoned Brock. Jesse knows everything that Walt did, because he was right there. He’s even the person most likely to sympathize with Walt, given their shared history. So, he truly understands Walt. When Jesse decides to take Walt down, this is the sum of all Walt’s evil actions converting his closest ally into his most dangerous enemy. Jesse turning against Walt is the ultimate chickens coming home to roost for Walt. Walt’s pursuit of his toxic, sociopathic ideals sowed the seeds of his own destruction, in Jesse. Walt’s own misdeeds destroy him, because along the way, they also destroyed Jesse, and they armed Jesse with the knowledge he needed to take Walt down.
TLDR: given their shared history, Jesse turning on Walt is the ultimate condemnation of his actions as well as the ultimate example of Walt’s own misdeeds coming back to haunt him, and so it’s fitting that Jesse is the person most responsible for Walt’s demise.