Jesse Pinkman never had a purpose. That's why he fell in with Walt to begin with. That's why he was so susceptible to Walt's emotional manipulation, why he ended up working with, and killing for, his former chemistry teacher. It's why he sat around in his house, numbing himself with pot, when it was all over.
In "Confessions", the eleventh episode of Breaking Bad's final season, Jesse found his purpose. For the first time, he had a reason to exist.
After learning that Walt had poisoned Brock with ricin, he knew exactly why he was alive. It wasn't to go to Alaska to start another life that would go nowhere; it was to go to Walt's house and try to burn it to the ground, to get some measure of retribution against the man who had poisoned his girlfriend's son and had held him as a psychological prisoner for so long.
Jesse had been set to leave New Mexico, thanks to Saul's guy who can make you disappear. Walt needed him to leave to further weaken Hank's already crumbing case against him. That all changed when he realized hat Walt had done at the end of Season 4.
Of course, prior to that big reveal, there was another bombshell.
Hank had failed to get any information from Jesse in the interrogation room. Saul helped see to that, as he marched in and shut Hank down before he could uncover anything that would help him bring down Walt.
Marie, perhaps innocently, or perhaps in attempt to play on Walt and Skyler's relationship with Flynn, asked Flynn to come over. Walt shut that down quickly, using the news of his cancer's return to manipulate him into staying away from his aunt and uncle.
Short on options and desperate to find anything to work with, Hank and Marie met Skyler and Walt for dinner. Their conversation was short, but Walt did slip Hank a DVD. The video contained a "confession" from Walt, one that framed Hank as the head of the meth operation. Because Hank had unwittingly accepted some of Walt's blood money to pay for his medical treatment, and because he hadn't yet gone to anyone else in the DEA with his story, there was more than a kernel of believability to the fairy tale that Walt blubbered into the camera.
Walt's world didn't end when Hank found out who he was; we saw another reminder of that in the opening sequence as Todd left Walt a message, informing him that he was running the old operation. Walt thought that life was going to go on, more or less peacefully, now that Jesse was going to be out of the picture and that Hank had effectively been blackmailed.
What he didn't count on was that Jesse would uncover the proverbial smoking gun.
For too long, Walt manipulated and marginalized his sidekick, doing whatever it took to keep him from complicating his life. Now Jesse is after Walt, and Walt knows, and if his digging into the soda machine for a gun is any indication, the two are now on a collision course, with just five episodes left in the series.