Norman Osborn, dying from the genetic disease in his one scene. The scene is shot in shadows, but his skin is deathly green, and his nails have grown to be practically claws.
From the score we have The Electro Suite. Its creepy whispery lyrics are scary as hell:
Bones are shaking / Madness breaking / I can feel the darkness taking over / DENYING / ARISING / THE CRYING / I'M DYING
The implication that the song's lyrics are what Electro hears in his head makes them more unsettling.
It first appears as he obsesses over Spider-Man, signaling how he's not all quite right in the head even before he gets powers.
The Electro Remix gives a few more creepy lyrics, too.
He lied to me, he shot at me, he hates on me, he's using me! He's dead to me! That Spider-Man... he is MY ENEMY!
The Green Goblin. He looks like an absolute maniac, and he has a very disturbing evil grin.
We have PLANS for you, Peter Parker!
Harry's transformation, for that matter.
The way Max becomes Electro. First electrocuted by joining two loose cables, then falling into the vat of electric eels, with the disconnected cables, leaking electricity into the water. With the twitching Max gives while he's getting a shock... It's quite unsettling.
When he takes over the city's electricity, the ensuing chaos sends two planes on a collision course! At the same time, the hospital Aunt May works at suffers from a power outage, taking out the life support systems and endangering many patients. Later on, as Spider-Man swings through New York, it's eerily dark—- save for a nearby building, whose lights are lit up strategically to form Electro's face as he stares at Spidey.
When Spidey and Electro face off at the power plant, following an impressive blow from the hero, Electro gets his second wind...and begins to viciously run Pete through with his electric body, flinging him all over the place while rebounding from various coils, the sounds of impact creating a G Minor version of "Itsy Bitsy Spider". Even though Peter cracks a quip about it ("I hate this song...!") the fact that Electro is deliberately playing such an ironic song while attempting to murder Spider-Man is downright chilling.
Gwen's death. Peter webs her, buts he's too late, even if he doesn't realize it at first. What's worse is you can hear the sound of her hitting the ground, so the audience can know that she's dead before Peter himself does.
The Amazon.com exclusive Blu-Ray set of the movie has the discs being stored in a creepy-looking bust of Electro's head. See ithere!
Cletus Kassedy even before he becomes Carnage, is a Death Seeker and Serial Killer. He targets criminals and has earned his status as The Dreaded solidified by the fact the criminal underworld fears him.
During the climax, he isn't satisfied with just him being Carnage, oh no, he decides to spread a symbiote to morph the other Ravencroft prisoners into a family.
During his speech to Spider-Man he is wearing a Slasher Smileeven when the symbiote isn't covering his face.