This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Twilight
Renesmee's birth. There's something uniquely unsettling about inserting a pregnancy scene comparable to the chest-ripping scene from Alien into a young adult romance. This includes ripping Bella's body (including spine!) from inside out. Not to mention that Edward, Bella's sweetheart, Jacob, Bella's friend, Carlisle, Bella's doctor, etc., consider and try toforce her to have an abortion.
Being made into a vampire isn't the mild process described in so many other works. Three straight days of burning, unbearable pain lie between human and immortality as the venom transforms the skin—and presumably internals organs—into a hard, marble like substance. Oh, and the reason why the vampires, even those who were dark-skinned in life are so pale...because the venom has literally seared the blood from their veins making them little more than empty tubes waiting to be filled with stolen blood. What somehow makes this even worse is that again unlike many other works, there is no involved, ritualistic process (such as a blood exchange) in becoming a vampire. All it takes is a single bite, no matter how shallow and you've joined the sparkly undead.
The mental state of the vampires. She doesn't seem to realize this, but Meyer made it quite nightmareish by saying that a vampire never advances mentally from the state they were in when turned. This means that Esme is still depressed over having lost her child and wanting to commit suicide, Rosalie is still in the mental state of having been recently raped and beaten by someone she loved and similar. For eternity.
Jacob's relationship with Renesmee is a bit Squick, but it starts to become more than a little horrifying when you think about how common the practice is among the were-wolves, and that adult men and older teenage boys are given unrestricted access to very small children they have romantic/sexualized feelings towards, changing their nappies and babysitting them and such. Or that the little girls, when they come of age, are expected to have sex with men they see as surrogate father or uncle figures. Even if they waited until the girls reached the age of consent, it's likely that the relationships would be more than a little coercive, especially as the men, who had been expected to sacrifice healthy adult relationship and even sex for their sworn love, might just get to thinking that the girl owed them something. At the very least, they'd probably have been gagging for sex for almost two decades. It doesn't exactly make for the gentlest introduction to adult sexual relationships.
The term is specifically called "child grooming", what pedophiles do to prepare a child for rape. Yeah.
The one girl we've seen who tried to resist her imprinter ended up with half her face torn off. Who's to say that won't be Claire in ten years time?
Perhaps even creepier is the fact that, according to cannon, Renesmee, despite her appearance, is not a child in terms of emotions, personality and mentality. She was, essentially, Born as an Adult, and is thus completely aware of her relationship with Jacob. It gets even scarier when you remember that imprinting effectively strips the werewolf of their free will, making them slaves to their imprintee. As noted by Bella, Renesmee sees Jacob as more an object than a person and is essentially using him as she pleases. It's not so much pedophiliac as it is domestic abuse from the baby's side.
Bella's blood is Edward's crack. Maybe you don't want to see Edward as a bad person, but he has a chemical dependency that he can't fully control and which, if he loses control even once, will result in the brutal murder of whoever happens to be nearby and tasty.
Edward was in Bella's room. Yes, it's presented as romantic, but let's look at this as we would in real life: he was in her room. While she slept. Without her knowledge. Repeatedly. And, as stated above, her blood is his crack. If his control were to slip, even for a second, she would be dead, and probably so would her father. This is something out of a horror movie. Actually, three-quarters of what happens in this series would have made an awesome horror series.
In fact, Bella being in love with Edward is the only thing stopping this series from being a pure horror story. Das Sporking did a "realistic retelling" of the restaurant scene where Edward confesses he's been following Bella and it's legitimately horrifying, even without touching on the worst aspects of the entire thing. Think about this; If Bella didn't love Edward back, what could Bella do about it? Edward is basically the ultimate stalker. He's faster than a car, can read minds (yours or those around you to find out where you are), and could probably track somebody by scent alone. If Bella was as creeped out by Edward's stalking as a real person probably would be, there is no way she could possibly get away from him.
For some, baby Renesmee. The CGI work placed it smack-dab in the middle of the Uncanny Valley.
However compared to the... abomination they were originally going with, which literally EVERYONE on set was terrified of, one can only thank the gods the movie's creepy CGI version was so tame by comparison. Behold CHUCKESMEE!
The series is also filled to the brim with Daddy Issues, feelings of worthlessness, and a extremely sheltered, almost childlike worldview and mindset.
When the Volturi ripped a vampire apart at the beginning of the New Moon film.
The Volturi's feeding methods. They have tourists visit the "beautiful abandoned castle" that they live in. The last room is the throne. There is no leaving. Cranked way up in the film when you hear the tourists screaming, but don't see anything. Spoiler'd because it will keep you from sleeping.
Even worse in the film is watching the tourists casually and happily walking in before it happens. As Heidi whispers to another vampire she passes about how she'll save some for him. Oh, and there are some kids in the group too.
Nightmare Retardant when you realize how incredibly unlikely it is that this business model would last for very long. People would quickly figure out that all these disappearances are linked to this castle. Plus, there's no one out there writing about it or reviewing it.
Imprinting. Special mention to Emily and Sam. Emily is biologically forced to be the "soul mate" of her cousin Leah's boyfriend Sam. He comes to Emily's house every day for weeks, begging to be with her, and every day she tells him to go back to Leah. Does Sam decide he He Wants His Beloved To Be Happy? Nope, he keeps coming back! Finally, Emily says Sam's just as bad as his cheating father ... and he wolfs out and mauls her face so badly that they explain it as a bear attack. Then they get married. Years later, the scars are still so bad that when Bella visits, she's warned not to stare. Emily is physically bound to her attacker, a man who could easily "wolf out" at any moment.
Imprinting is horrifying from both ends if you stop to think about it. The child grooming and poor Emily's predicament are both obvious, but for the imprinting wolf, their own genetics have just mind raped them into orbiting around this girl or woman forever, wanting nothing but them, needing nothing but them, theoretically at least changing whatever is necessary to keep them. In Sam's case the need overrode already-existing love and his self-control, so that he ended up nearly killing Emily (and theoretically that risk may make their relationship a mutual nightmare of fear). Jacob outright hated imprinting and pointed out this aspect but bam, one look at Renesmee and that never mattered.
Alice. She was institutionalized as a human because she saw things. When she becomes a vampire, her powers are much more useful. But she has to live with that knowledge of the future (futures) whether things turn out that way or not, including the visions of her father, lover, and various allies dying in battle against the Volturi in the last film.
Isn't that just a visual version of mundane inference? Like tossing a coin and seeing "visions" about it being either head or tail?
The sheer lack of free will that werewolves have over their own lives, from the trauma of becoming a werewolf to the inherent squickiness of imprinting, is horrifying and depressing. With imprinting specifically, Meyer's laziness in writing took the concept of "soul mates" and took it to its squickiest rock-bottom. The fact that she tried to explain it away in-story worsens both the nightmare fuel and Unfortunate Implications.
Rosalie's back story: beaten, gang-raped and left for dead by her drunk fiance and his friends, she is transformed into a vampire and takes revenge on them in a wedding dress.
And Carlisle simply picked her up from the street, took her to his home and bit her. Didn't explain anything, didn't give her an option and his reason for turning her was because she was so beautiful and he wanted a mate for Edward. He didn't necessarily wanted to save her. It doesn't help that Rosalie must be scared out of her mind after being raped, beaten and then picked up by some other man. And Edward and Carlisle talk about her and not to her the entire time.
Rosalie's reason for turning Emmet. He reminded her of Henry, her best friend's baby. She turned him, because he reminded her of the baby that she wanted but never got... and she's sleeping with him.
Thinking about it, how Bella's life as a vampire would go. Bella never had any real hobbies, no extracurricular activities, didn't seem to care all that much about her job, had no ambition to study anything, either in school or going to college and has no idea what to do with herself if Edward is not around. What will her existence as a vampire change? What will she do if Edward goes off on a hunting trip without her? She has nothing to do.
Vampires babies. They're vampires so they are strong, fast, and almost indestructible. They're babies so they don't grow out of their newborn phase (eat every human they come across) and you know what's worse ? They're so cute that any vampires that sees them immediatly have to protect them and to help them kill people to the extent that the Volturi had to kill everyone implied in their vampirisation and made the subject taboo.