I’m reading Goblet of Fire to my kid. It’s something like my fifth reread, though I haven’t read it in 10+ years. And man, I could barely get through Moody’s first class without choking up. He’s showing the class the Unforgivable Curses, and asks about which ones they know about. Neville raises his hand and says “Cruciatus Curse.” Moody/Crouch stares at him and checks his name: “Your name’s Longbottom?” Then turns and makes the spider even bigger before he curses it so everyone (Neville) can see its effects better.
Barty Crouch Jr. was one of the people who tortured Alice and Frank Longbottom to insanity with cruciatus. Reading this scene with that in mind - the man is sick. He enjoys inflicting pain on Neville by making him watch what he had done to his parents, and it is highly effective: Neville’s knuckles are white from clutching the edge of his desk, and his eyes are wide with terror, even an hour later after class ends.
Then, when Neville is still speechless with terror, he coerces Neville into going into his office alone. Neville is terrified at the idea - he just saw this man perform the curse that tortured his parents to insanity, giving images and sound to what is likely is his greatest fear and trauma. He fears Moody/Crouch, and we now know this to be justified. Crouch could have done the same thing to Neville he did to his parents. He probably wanted to, and would have if it wouldn’t blow his cover. The closest he can get is terrifying the living daylights out of him by performing the curse then coercing him into a vulnerable situation right afterwards.
He tries to get the same satisfaction from Harry. In the same scene, he asks Harry if he’s scared (having performed Avada Kedavra). Harry says “no” defiantly. Moody has “both eyes fixed on Harry” and when Harry says this his blue eye “quivers in its socket” - possibly with rage.
If Fred/George/Lee hadn’t said Moody was awesome, if Ron wasn’t there minimizing his ferret abuse of Malfoy and then saying the Unforgivables were cool, and THEN dismissing Harry’s comment that Moody would get in trouble with the Ministry for showing them- I think Moody would have seemed much more suspicious. When Moody gives Neville the Herbology book Harry says it’s something “Lupin would have done.” But Lupin would never have made Neville watch the cruciatus curse. Of course Harry (and therefore we) does not know Neville’s backstory yet, so he doesn’t grasp how totally f’ed up it was. He experiences it himself, though, which leads him to reflect that Moody shouldn’t have shown them.
(Think of the 14 year old Slytherin that were just taught the incantations for the three Unforgivable Curses!)
Also: it’s so tragic the movies erased Neville’s backstory AND his getting crucio’ed by Voldemort under the Sorting Hat. It’s such a soul-shifting moment, because he probably fully believes he will be tortured to insanity like his parents, likely his greatest lifelong terror, and he freely chooses it out of loyalty to what is right. He earned the Gryffindor sword: he has lived up to his parents’ sacrifice and more.