Walt strangling Krazy-8 with a bike lock. Although the way he deduced what he'll do was awesome, it still gives a scary glimpse of Walt's inner ego, Heisenberg.
Tuco punching his henchman to death for talking out of turn.
Heck, him nearly beating Jesse to death is pretty shocking as well, especially since it seemed like Tuco would act compliant before.
"Negro y Azul". "What's the matter Schrader? You act like you never saw a severed human head on a tortoise before! Then KABOOOOOOOM.
The couple who rob Skinny Pete in Breakage and own the creepy, disgusting house in Peekaboo. Doubles as a Tear Jerker since the couple has a small child who lives in these terrible conditions and is blissfully unaware of it.
That horrible noise when the woman drops the ATM on her husband's head...
Jane's death. Holy crap. It comes out of nowhere, with Walt attempting to wake up Jesse and accidentally moving Jane onto her back in the process. We then get to witness 30 seconds of Jane spasming and choking on her own vomit, while Walt just stands there, unable to do anything.
Unwilling, more likely.
The Cousins. Their stoic nature, their merciless murders, and their thirst for revenge for the sake of family... both packaged with a giant silver axe. These are two people you do not want to cross.
In "Rabid Dog", Jesse firmly establishing the outside view of Walt as The Dreaded to Hank and Gomez. Considering Jesse originally thought of Walter as nothing more than an annoying Jerkass teacher who randomly decided to break bad, this is pretty unsettling.
Jesse: What if it's about... killing me? Y'know, getting me out in the open? Hire some uh, some clock tower guy or have me sit on a poison needle, y'know?
Hank: Nothing's going to happen to you. I mean, the plaza is one of the most wide open public places in all of Albuquerque, it's the middle of the day and Agent Gomez and I will be there with you.
Jesse: Look, you two guys are just... guys, okay? Mr. White? He's The Devil. Y'know, he is-he's smarter than you, he's luckier than you. Whatever you think is supposed to happen, I'm telling you, the exact, reverse, opposite is gonna happen! Okay?"
The shootout at the end of To'Hajiilee. Holy shit!
Indeed, Walt himself is a walking, breathing Nightmare Fuel. Beneath that mild-mannered teacher personality lies a murderous sociopath willing to commit capital crimes without a second thought. And remember that all this takes place in just one year.
Almost every minute of Ozymandias:
Hank's death, with Walt crumpling to ground beside him.
The fate of Jesse, who is tortured and forced into slavery by Uncle Jack and Todd.
Given how far the show has been willing to go before, you spend the whole scene terrified that Skyler, Walter Jr., or even Holly is going to end up with the knife in their gut.
Just the shot of Skyler and Walt Jr. cowering on the ground in front of Walt, one of Jr.'s arms flung protectively in front of his mother, looking up at Walt like he's some sort of monster- which by this point, he is.
Skyler's scream after Walt drives off with Holly.
Marie's face when Walt confirms Hank's death.
The ending of "Granite State", when Walt hears the Schwartzs badmouthing him on TV. He instantly turns from depressed and suicidal, to determined. It's like a non-physical Dr Jekyll And Mr Hyde transformation. All with the extended Breaking Bad theme playing. Heisenberg is back!
The worst part of "Granite State" was Skyler being confronted by Todd and two Neo-Nazis in balaclavas in Holly's room. Todd threatens her and her children, all while remaining eerily calm and polite.
Todd Alquist is a Nightmare Fuel made flesh. How bad? Not even Heisenberg is as cruel as this man. His second appearance is him shooting a kid on the spot, without warning, without so much as asking his bosses' permission. Then he beats Jesse to a pulp, imprisons him in an underground dungeon, and enslaves him to meth-cooking, all while holding Andrea and Brock, two innocent civilians hostage. When Jesse tries to escape, instead of shooting him just as Jesse begs him to do, he shoots Andrea as a warning while leaving Brock alive as a leverage. And all after giving him ice cream for a good day's work.
The Neo-Nazis. They're...they're just too good at what they do. Kill ten different people all locked up in prison? No problem. Murder two DEA agents and bury them in the desert? Done. Torture a man and enslave him to force him to cook meth? Piece of cake. Break into a woman's house to blackmail her despite the fact that there are police watching the house and getting away without being caught? Did it already. Murder a woman directly in front of her house and saying they'll have no problem tracking down her son and doing the same to him? They'll do it. All of this alone is bad enough, but the fact that at time of writing before the last episode so far the Neo-Nazis are 110% Karma Houdinis not a single one of them dying, all of their operations going smoothly, and to top it off they have 70 million dollars to show for their work And you thought the Cousins were bad.
Further Nightmare Fuel: This is Truth in Television. Aryan Brotherhood, the group Breaking Bad's Neo-Nazi is based on, is the most vicious prison gang in the United States. Though they only make up 0.1% of prison population, they are responsible for up to 20 percent of murders in the federal prisons.
Thankfully they all die in Walt's killing spree. Still though, they're not exactly pleasant people.
Walt's homemade chemo treatment. People with a fear of needles should stay far away.
How Walt ensures Walter Jr will receive his inheritance. He breaks into the Schwartz home, looking at their belongings while Elliot and Gretchen cook and banter unaware. He then has Badger and Skinny Pete use laser pointers on them, making them think that they are in the sights of two snipers, and convinces them that they'll live under constant supervision and fear until Walter Jr receive Walt's money through them, possibly for the rest of their lives as well.
Walt's "visit" to the Schwartz's could have been lifted straight out of The Strangers: he effortlessly stakes out their pricey hilltop mansion, slips silently inside, and then bars the doors. He's in the house for several minutes before they even notice he's there....
The nauseating sound of Todd's neck snapping.
From Lydia’s perspective at the end of Felina: Imagine calling Todd to ask if Walt had been killed, only to hear from Walt himself that not just Todd, but his entire gang is dead and that cold you thought you had is actually an incurable poison that he slipped into your favorite tea sweetener.