This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Iron Giant
20 feet of very angry robot
If the deleted dream sequence is of any indication the Giant is one of many machines sent out across the universe to singlehandedly obliterate entire planets or, worse yet, may have already done such atrocities in the past.
Extra Fridge Horror in that the other robots might still be out there. And they might find Earth. And not be as friendly as the title character.
Given that the reason the Giant is good is due to Amnesiac Dissonance after hitting his head, as well as his friendship with Hogarth... how do we know that he wasn't returned to factory settings and his memory wiped, after he was hit by the nuke? Oh, Crap!.
Even better (worse?), the 2015 "Signature Edition" adds the complete version to the film, and it is horrifying. For starters, the final animation shows the army of giants walking amongst the ruins, and the fire present gives it a very Hellish appearance.See here for the complete scene.
The fact that he just strolls after the soldiers fleeing for their lives, blasting everything in his path.
The entire scene is nightmarish for a completely different reason; we're seeing a loving soul willingly succumbing to his murderous side because he thinks his best friend just got killed. Adult Fear at its finest.
Mansley chloroforming Hogarth. It comes out of nowhere, and it's downright cruel.
Even worse is his Moral Event Horizon shortly before that. Up until this point, Mansley has been somewhat of a bumbler, and a lot of fun to watch as he repeatedly fails to catch the Giant. And then he very calmly threatens to make Hogarth's life such a living hell that he'd eventually be taken away from his mother if he doesn't reveal where the Giant is hiding. The worst part is the quiet, gleefully sadistic tone he's speaking in the whole time- he is perfectly capable (and willing) to follow through on any threat he makes, and he loves it.
General Rogard: That missile is targeted to the Giant's current position!Where's the Giant, Mansley?!
Kent Mansley:[Upon realizing the Giant is right above them] Wha—? Ooh... We can duck and cover! There's a fallout shelter right there, if we—
General Rogard:[Angrily] There's no way to survive this, you idiot!
Kent Mansley:You mean we're all going to...
General Rogard:To die, Mansley. For our country.
From Hogath's point of view, Mansley blowing up at him in the ice cream parlour. Think about it, Hogarth may be a strong kid, but he's still a kid and here he is alone with this complete stranger to whom his mother recommended a tour around town. Even if he is in a public place, this boy is in a very vulnerable position of having this man who up until this point, Hogarth and the audience has believed to be a pencil pushing bearaucrat, going off on a near-psychotic rant. Thank God for Landslide!
The Iron Giant's first appearance at the power plant.
The bit from the old horror movie Hogarth watches where the mutant brain is presumably eating the man's head.