"Alan York" coming to see his daughter in the hospital and then smothering her to death as she struggles for a good 45 seconds.
Jack's towel torture threat.
David Palmer's near Unstoppable Rage when he learns that Sherry leaked to the press that he was still alive and in turn endangering Kim Bauer's life, leading him to nearly hit her until Mike Novick is able to calm him down. Let me say that again: Senator David Palmer, someone who's usually one of the most collected characters in the series, outright threatening to use physical violence against someone. Even if it was Sherry, seeing him nearly resort to something like that can be startling, especially the second he angrily screams her name after learning what she did.
Then there was the poor PI who working with Kate Warner. And like Jack's torture above, he was stripped bare naked. Then Sayid Ali's men uses a buzzsaw onto his back, and you can hear it shredding him. OH... MY... GOD.
Then there's the nasty liquid torture that opens up season 2. Yikes.
Jack carrying around a decapitated head, wrapped around a white sheet, to gain allegiance to a local gang who are loosely related to the terrorist threat.
Jack threatening to kill Sayid Ali's two sons if he does not tell him where the nuke is located. Even President Palmer thought this was a move too far, and didn't allow it, but Jack went ahead and went through with the threat anyway. The audience initially believed that Jack ordered one of the sons to be gunned down... on camera. Fortunately, the whole threat was a facade, and the bullets were blanks. Still, the very thought of Jack being pushed to the point of murdering innocent children, simply because of their father's extremist behavior, was horrifying.
It's fair to say Season Two had many horrific and extremely gory scenes, even by 24's sadistic standards. Given how much season 6 was criticized for its violent torture scenes, it's amazing how none of the detractors brought up any of these scenarios.
All of the scenes involving the Chandler Plaza Hotel. Imagine waking up from a nice vacation at four in the morning with an unstoppable nosebleed. You come down to the lobby only to find other guests suffering from the same symptoms. A panic breaks out, a guest who tries to leave is shot dead, and news breaks out that there's no cure. The only thing you can do is stay in the hotel and either let the virus slowly kill you or take a suicide capsule.
Even after capture Saunders refuses to reveal where the virus is. So Jack forces his daughter to be led into the hotel so she'll infected. A triple whammy between being forced to go that far, Saunders' daughter screaming like she was in every horror film ever and Jack's vicious and angry disregard in the face of Saunders' reticence.
After CTU took too long to find and apprehend the terrorist responsible for the virus mentioned above, Jack was forced to gun down his boss on the wishes of said terrorist.
Chase having his hand planted on the wall by a terrorist's pistol, and then the terrorist fired the gun point blank.
Chase getting his hand chopped off by a fireman's ax, just to remove the virus attached to his wrist. Cue extreme pain and Chase passing out from the trauma.
Habib Marwan escapes by impersonating a CTU Agent; when a hapless agent realizes his buddy isn't who he appears to be, Marwan pushes his arm up, sticks his gun inside the agent's vest and puts three rounds in him. Horrible.
Marwan's Slasher Smile as he willingly dies in order to keep Jack from stopping his plans is... disturbing to say the least.
Cheng Zi, the MSS officer and head of security at the Chinese consulate. His effortless kidnapping of a CTU agent is very unnerving.
Audrey Raines' liquid copper torture and subsequent Heroic BSOD
Jack's cold and clinical way of instructing a young naval engineer to slit the throat of one of Bierko's goons.
"When you get within 18 inches of him, grab him by the mouth, pull his head towards your chest and cut his throat deep enough to sever his vocal chords and his carotid artery."
Just seeing what happens to the victims of the Syntox gas. One particular horrifying scene is after it's successfully released in CTU, most of the main characters and a few minor background ones are able to escape inside a sealed room. However, one female worker isn't able to make it in time and starts begging everyone to let her in (which they can't, since that would contaminate the room) and eventually succumbs to the gas.
Cheng Zi and his MSS officers who come out of nowhere and catch Jack Bauer with his guard down, putting him on what must have been the most painful Slow Boat To China in recent memory. And then the cruel torture and trick he played on Jack.
Abu Fayed torturing Morris with a power drill. He comes very close to drilling a hole right through his head!
Cheng Zi again. Whether it's his torture of Audrey or the resources he uses to leave CTU in the dust, he's gotten even more scary since season 4.
Jack's reaction after finding out his brother Graem was behind the deaths of President Palmer, Tony, and Michelle.
The opening scene where the Child Soldiers are encouraged to execute the man.
Jack and Tony gassing Ule Motobo and his wife out of a panic room using household chemicals, especially considering that you see them choking on the gas until Mrs. Motobo opens the door, unable to watch her husband die.
Renee Walker being buried alive. The very last thing the audience is treated to is a point-of-view shot of dirt covering her face, followed by a silent clock. And we don't know if she lives or dies until the next week. ( She is quickly saved and revived.)
Larry Moss's death as he gets suffocated by Tony. The look of horror in his eyes as he realizes what's happening and as he slowly dies is one of the darkest moments on the show.
Renee's solution to getting Ziya's parole bracelet off, sawing his thumb off!
Renee's descriptions of Vladimir Laitanin and the way he treated her could also fall under this.
Jack's Cold-Blooded Torture of Renee's assassin. Made even more disturbing by the fact that this was the show's hero doing the torturing.
We see the aftermath of it too, and it's just as frightening. Jason Pillar (rightfully) points out that Jack didn't just kill the guy, he butchered him. It looks like the guy's been torn to pieces by a wild animal.
Jack mounts an armored assault on former President Charles Logan's car. We see Jack from far away, methodically shooting his way throughLogan's secret service agents, Logan for perhaps the only time in the series looking horrendously afraid. Jack makes his way to the car and just stares through the window wearing this◊.
Seeing the aftermath of "The Massacre."
Jack's execution of Dana Walsh. Although Jack has killed others in cold blood before and Dana has been working with the antagonists, just seeing it carried out is so brutal that it makes you really start realizing that Jack's dark side hasn't just surfaced again, it's finally taken him over.
President Taylor's attitude change in the 8th season is very nightmarish without having any violence or gore directly involved. She initially seems reluctant to be following Logan's advice, but easily starts accepting it more and more as time goes on. Then the ball drops and she threatens to declare war on Kayla Hassan unless she agrees to sign the treaty, all on her own choice. Seeing a character who had originally been one of the most moral and with some of the best judgement go down this route with so little trouble is a very disturbing picture of how even one of the most noble souls can become tainted.
Just the thought that Jack was mere seconds away from pulling a trigger that would kill the president of Russia and thereby start World War 3 as a result. The series has made a big deal of preventing something like that from ever happening because of the consequences (the biggest case being in Day 2) yet it's the protagonist of all people who's come the closest to achieving that. Damn.
Live Another Day
Chloe's very first appearance, with her in CIA custody and screaming in agony as she's injected with hyoscine-pentothal. It's very abrupt, and is likely to take someone off guard.
The sudden betrayal by Simone to Derrick Yates. We see her stab him right in the ear with no Gory Discretion Shot.
When Naveed refuses to cooperate with Margot anymore, she responds by having one of her own daughter's fingers cut off. And Simone's 100% loyal to her and hasn't failed her in any way - it's just because she has the misfortune of being Naveed's weakpoint, so Margo is determined to exploit that in any way possible.
Again with the torture. This time Kate Morgan's the victim. Stuck in strappado, subjected to electric torture, and menaced with a power drill. Also, the last thing the torturer said before he was interrupted was "now for your eyes"
The results of the hijacked drone Margot and Ian used to blow up some of the hospital. Among other things we see a woman who didn't escape the blast and has now lost an arm in the explosion, screaming in agony, while there's also a paramedic now caught on fire and burning to death.
Chloe and Cross head back to their group to find that every single other member of their group has been mercilessly slaughtered by Cheng Zhi.
Jack's rampage upon learning of Audrey's death and Cheng's death is extremely setting. After Jack goes into Terminator mode and kills every single bad guy in his light of sight, beats Cheng to near death, and gets him to video himself as the mastermind of that day's attacks, what does Jack do? He hacks off Cheng's head with a katana with one go.