You could say the transformation from Chemistry High School teacher to Scarface (as creator Vince Gilligan has said is the narrative structure all along), started in Season 1 when he blackmailed Jesse into cooking meth with him. And there have certainly been key plot points along the way, sealing his status as a criminal and killer and pushing him further into a new identity (choking the guy in his basement with a chain, allowing Jane to suffocate on her own vomit, shooting the drug dealers on the street point-blank) but even as the last episodes of Season 4 unfurled and he was orchestrating Gustavo Fring's death, I rooted for him. I had shifted my allegiance towards Jesse after seeing all he had been through--the ups and downs of addiction, and the way he always ended up in the middle of Walt's convoluted schemes to remain on top. Jesse annoyed me at the beginning of the series, but he matured and became lovable--he's the only one who has morals, who cares if people die, and his love for children is endearing. But Walt's arc has shown an opposite set of traits. Although he started out wanting to make meth to support his family, the money--and more so the power--the meth game has given him has gone to his head and he's became a homicidal egomaniac. It wasn't until this first episode of Season 5 that I felt I feared Walt. I was waiting to see if he really had poisoned Brock. I could understand wanting to kill Gus because his life, Jesse's life and their loved ones depended on it. But that last shot in the last episode of Season 4, when the camera pushes into his backyard and we see the Lily of the Valley plant and we realize Walt manipulated Jesse and banked on his love of children by poisoning Brock was beyond diabolical. He said to Jesse "Who do you know who has no problem using Children?" He meant Gus, but by his actions, he also meant himself. He killed Gus, only to take his place.
I still held out hope that maybe Walt didn't really poison Brock, maybe it really was Gus, maybe it was just a mistake, maybe there was some plot twist that would blow our minds when Season 5 started. But the fact is that Walt is a killer, who unlike Jesse, has no loyalties to anyone but himself and his family--and even that has become questionable as we see Walt in the Season 5 premiere hugging his frightened wife and telling her he forgives her for having an affair with Ted and giving him Walt's money to pay off the IRS. He's not sincere--there's a look on his face of that same manipulation he's used with Jesse and everyone else who's stood in his way. He knows she's scared and he's using that to his advantage. Everything has changed. Saul is scared of him. Even Mike listens to Walt and helps them destroy the last piece of evidence from the meth lab. The premiere episode has set Walt up for his final 16 episodes of the series and the final obstacles and steps needed to be a full blown, hardened criminal. Cable television has become the new "indie film"---the fact that AMC has allowed Vince Gilligan to take a character and turn him "bad" (thus the title) and end it after 5 seasons despite being called "the best show on television" in the name of artistic integrity is admirable. The hard part now is having to wait a week between episodes. It leaves me too much time to guess how Walt and Jesse will end their last meth deal. For Jesse, I hope it ends well. As for Walt...well, we all know how Scarface ended.