This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Spider-Man 3
Maybe this scene was deleted for good reason.
After being forced off of Peter, the symbiote latches on to its next host with a vengeance. Seemingly in order to ensure that it won't be rejected again, it goes all the way with Eddie Brock, literally pouring itself down his throat. But by far the creepiest part of that scene was when Brock started to enjoy it.
VENOM!!!◊ It's hard to believe the same actor who played Eric Foreman is underneath that suit!
Also when Eddie gets pulled out of the symbiote in the final battle. Not that bit, but when the symbiote starts to grow taller, towering above Peter, nearly reaching the roof... that's when you realize that THIS is the true Venom.◊ It's a good thing Carnage was never brought into the film, because that would have been absolutely terrifying.
A particularly horrifying version of this scene occurs in the novelization: When Peter finally forces the symbiote off Eddie, it's revealed that the symbiote had, at one point, absorbed Eddie's body to create one for itself, leaving only his smoking bones behind. Then the symbiote talks, telling Peter to never wound what he can't kill. It's left ambiguous how much of Eddie was coming through the symbiote during the final fight, or if it was the symbiote talking all along. Yikes.
The beginning of the final one-on-one battle. Venom screeches in the background, crawling around the construction site like a lion jumping around its prey, as Spider-Man cautiously looks around, looking positively goddamn terrified the whole time. And then...Venomrandomlygrabs Spidey from directly above him. While this Venom may be a divisive one, that scene helps evoke the fear Venom can spread, in universe or out.
The symbiote attaching itself to Peter as he lay asleep and defenseless.
In the editor's cut, just before Peter humiliates Eddie at the Bugle, he is in his apartment, staring at the black suit. Then he begins to smile eerily as the music playing gets louder to the point you'd think the symbiote was talking to Peter.
The editor's cut also includes some slight re-structuring of events and more shots of Peter putting the black costume back and then putting it back on later, making it much more apparent that it's like a drug he is becoming increasingly addicted to, and he's getting worse each time he puts it on.
Peter Parker as Black Spider-Man is pretty terrifying in his own right, as his fighting style is more aggressive and dirty and he can withstand Sandman whereas in the first fight he was overpowered.
His attempted murder of Sandman, especially his chilling delivery of "Good riddance."
The infamous deleted "mirror" scene (shown right), where Peter stares at his own reflection, only for him to get a brief flash of the symbiote's true nature in the form of a Jump Scare.
Harry's half damaged face from Peter, his own friend, throwing one of his bombs back at him.
Related to this, the editor's cut somehow makes Peter's infamous goofy dancing montage darker by re-arranging its order, placing his humiliating of Eddie Brock before his confrontation with Harry, which now has a new beginning to it where Peter is already in Harry's room, waiting for him and saying "I'm going to enjoy this", showing that he intends on fighting Harry rather than just doing it because he lost his temper like in the theatrical cut. He then blows Harry up with his own bomb, walks away without looking back...and then immediately cut to the silly montage, which now plays like Peter just giving absolutely no fucks at what he just did to his former best friend.
In the video game adaptation of the film, the Lizard levels are this, full stop. In them, the player is treated to the horrifying sights of how the Lizard has created his own reptilian-army — by kidnapping humans from off the streets and placing them into cocoons that slowly mutate them into lizardmen. Things only get worse later on in the game when Spider-Man revisits the sewers, finding that it has now been completely taken over by the Lizard and transformed into a location that feels like it came straight out of a survival horror game (right down to the level's music).