The very concept of infection. Can occur in as little as 10-20 seconds and requires only a small droplet of infected blood.
This on top of spending 10-20 seconds being brutally beaten to death by an infected person before you join them.
Or having a motorcycle helmet or something on, keeping you from being infected until your skin has been torn apart by the force of their blows.
Guess what? You just swallowed a drop of infected blood! Now you have ten seconds to live before you become a mindless zombie unless one of your friends hacks you into pieces with a machete. While your mind is still intact and you are fully aware of it.
Say you don't get outright attacked? Well, just getting infected is extremely painful all by itself. So you get to spend your final moments screaming and/or convulsing no matter what!
The whole sequence where Jim wakes up and finds London deserted and destroyed is a potent dose of Nightmare Fuel in and of itself. Waking out of a coma to something like that, with absolutely no idea what's going on...brrr.
The London sections are made so much worse if you live in and around the city. Seeing streets and buildings you know just abandoned or full of pseudo-zombies is really un-nerving.
The bit when Jim goes into the church and he passes a huge part of graffiti saying 'THE END IS EXTREMELY FUCKING NIGH' and finds the entire floor of the church covered in bodies but when he softly says "hello", some of them sit up and stare up at him with gaping mouths and those vacant red eyes.
The dinner scene. The entire room is full of rotting corpses, including the mother holding onto her baby. With each passing second, Jim himself realizes it wasn't just stupid to get inside, but he will probably never be able to wipe that image from his memory now.
And then the infected boy attacks him, uttering the only line spoken by infected and forcing Jim to bash his head open, despite being relatively easily subdued. The single line said by him and his general defenseless makes the kill all that more impactful.
Mailer. Particularly in the shot where one of the soldiers looks out the window and finds Mailer staring right back in at him.
Those soldiers. There's one scene where Hannah and Selena are sitting on a small couch, terrified, being slowly circled by a bunch of rapist soldiers who are probably mentally drawing straws to see who goes first...and they dressed the girls in ball dresses. It's bad enough that they're tricking desperate women into being their sex slaves, but they're also trying to pretty them up first. Why would they add that extra disturbing dimension to what already is a disgusting horror?
Sadly related, the scene where Selena is desperately feeding Hannah as much Valium as she can 'so she won't care'.
The reasoning only adds to the horror: They're trying to desperately imagine some sense of civility to it. Remember the original reason that West promised them women: not for sexual release, but for children. For some sense of a future.
Made more horrifying by the fact that one can see how this would happen. As far as they knew they were the only living humans in Britain. It's not implausible that nine men, having seen the initial effects of the outbreak and then been holed up with each other for a month, might start to lose it.
The real nightmare is the realization it took them a month to reach that state, from what was passable and adjusted to preparing a gang rape of a girl that can barely be even called a teenager.
"In the House - In a Heartbeat". The alternating piano notes, the guitar chords, over and over again...an Ear Worm made of Nightmare Fuel.
When Frank is infected and urges Hannah to stay away.
Frank's infection in general, and its implications about the Rage virus. The body had clearly been there for awhile. His only exposure was a single drop of infected blood in his eye. And the virus still takes effect within 30 seconds. Exactly how long can this thing survive without a living host?
The Rage infectees slowly dying of starvation
How about the opening? Sure, the activists were Asshole Victims, but it's still quite terrifying.
Patient zero herself; she's immediately reduced to writhing on the ground, screaming that she's "burning", before accidentally spitting blood at one of her friends, infecting him. By the time the scientist comes back in to kill her, the other infected activist is gone, she's already killed the remaining activist, and then she turns around to face him...