Q:

Why did Crouch have Krum torture Cedric instead of just stun him?

In the Harry Potter and the Goblet of Fire, Barty Crouch Jr. as Moody tries to get Harry to the cup first by taking out the other champions. He says that he stunned Fleur and then had Krum take out Diggory. This seems illogical, though, because Harry heard Cedric being attacked and tortured and saved him, which almost made Harry not touch the cup first.
Why not stun Krum and Diggory like he did to Fleur?
Why risk the sounds of torture? Why imperius Krum at all? I know that Crouch was evil and sadistic, but he was intelligent and needed the plan to go smoothly. It doesn't make sense.

A:

My own belief is quite simple: Barty is a Death Eater. One of Voldemort's most fanatical, crazy followers (one of the top three, behind Voldy and Bellatrix). He does evil for the sake of evil, and he cannot think of evil that is not bombastic, not even if practicality would get him better results.
This is an issue that Voldy himself has, as pointed out by Dumbledore. The best example of course being the plan with the Cup being a PortKey and trying to humiliate Potter via wand duel versus just killing him immediately.
Forcing a person to torture another person is right up their alley.

A:

Fairly sure there is no canon answer, but I can make a guess:
To punish Karkaroff for abandoning Voldemort.
When he’s interrogating Harry, he speaks of his contempt for people like Karkaroff (Death Eaters who abandoned the true cause), and how he wants to see them punished:

“I told you, Harry… I told you. If there’s one thing I hate more than any other, it’s a Death Eater who walked free. They turned their backs on my master when he needed them most. I expected him to punish them. I expected him to torture them.”

He likes taking matters into his own hands – the Dark Mark at the World Cup, or killing Harry when Voldemort failed. I think he’d have hated seeing Karkaroff so gleeful in the Tournament, and wanted to humiliate him. Having Krum use an Unforgivable Curse in the maze is a good way to do so.
Had all gone to plan, only Harry would have reached the Cup and been killed. Then, as a bonus, Cedric would have testified to Krum’s use of the Cruciatus Curse in the maze.
He’d find it difficult to prove he was acting under Imperius, so he’d face a life sentence in Azkaban (or the equivalent punishment in Bulgaria). His actions aren’t out-of-the-ordinary for a Triwizard Champion. As the Durmstrang Champion, it would embarrass Karkaroff, and raise serious questions about the suitability of teaching Dark Arts to teenagers.

