This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Home Alone
The wreath of the McCallister residence at the beginning of the movie features a rather mean looking Santa, with his eyes in shadow Fist of the North Star style... and then they zoom in on the thing! Yeesh!
The furnaces "HELLOOO KEVIIIN!!" Doubles as in-universe Nightmare Fuel for Kevin.
Nullified by the fact that Kevin simply tells it to shut up, and it works.
The basement. Kevin goes down to see if any of his family hiding there. Among the objects is a upper part of a female mannequin.
Marley, another in-universe horror for Kevin, as evidenced by his scream when they run into each other outside Kevin's house. Thankfully, Kevin eventually becomes friends with him.
Buzz's pet tarantula, Harry and Marv to some people, and a few of the booby traps.
Highlights of the booby traps: the nail through Marv's foot, the tarantula, the glass ornament stepping, the scenes with the furnace, and the electric sink. That wasn't Kevin down there in that scene...that was Henry Evans!
When Harry and Marv manage to catch Kevin, they gleefully claim that everything Kevin did to them, they're going to do to him. All those booby traps? Not so funny when you picture those things being done to an eight-year-old boy. Harry even said he was going to bite off all of Kevin's fingers one at a time, and had every intention of going through with it. He actually had one in his mouth before a shovel-wielding Marley came in to rescue Kevin.
Near the end, when Harry and Marv are standing over Kevin, paint all over them looking like dried blood, crazy slasher smiles...and the line "My, how the tables have turned". Brrrr.
Harry comes very close to shooting Kevin in Central Park. While the gun did turn out to be jammed, it's still a very unsettling scene of Harry wanting to savor killing a kid. On top of that, the way the gun sort of oozes when he takes it out of his coat isn't exactly a pleasing image.
The Oh Crap! moment when the now Sticky Bandits see Kevin again. When Harry greets Kevin with a very Faux Affably Evil "Hiya paaaal!", Kevin turns around and to see the two towering over him with giant Slasher Smiles.
And when he rises his hand towards Kevin, revealing the burn in the shape of the doorknob in the first movie.
Keep in mind that based on some of the comments they made, Harry and Marv intended to kill Kevin from the moment they saw him in front of the toy store - and they probably had something more grisly in mind than just shooting him. Marv, for example, implies that they could drown him, and Harry mentions something about a subway tunnel.
The Pigeon Lady standing and watching as her pigeons swarm Marv and Harry, especially when she laughs.
Marv getting electrocuted and turning into a screaming skeleton for a few seconds was enough to scare the crap out of you.
The Cab Driver in the second film. Especially that creepy shadowed face of his. Double points for showing his face directly into the camera.
Not to mention Kevin walks past a couple of hookers- who come on to him! (Hey, lookin' for somebody to read you a bedtime story?)
The first trailer for the movie includes a creepy CGI Statue of Liberty that suddenly screams the same way Kevin does in the first movie.
Each and every one of the concierge's Slasher Smiles, especially the one when Kevin is trying to get away from Harry and Marv by heading to the hotel, and the camera lingers on his face for a few seconds.