It may have been part of Krum’s “instructions” to use Crucio.
Victims of the Imperius Curse feel internally relaxed, and have no emotions of their own. However, the person who cast the Imperius Curse could probably command the victim to feel certain emotions if they needed them to. From what we see of Krum’s actions in the maze, Barty Crouch Jr. could have simply commanded him to cast Crucio on the other Triwizard champions (presumably except Harry). We see him using Crucio on Cedric, and Cedric said he heard Fleur scream, so it’s possible casting Crucio could be a direct command.
“Did you hear Fleur scream earlier?’ said Harry.
‘Yeah,’ said Cedric. ‘You don’t think Krum got her, too?’
‘I don’t know,’ said Harry slowly.”
- Harry Potter and the Goblet of Fire, Chapter 31 (The Third Task)
Under the Imperius Curse, it might not matter what Krum’s emotions are. If Barty Crouch Jr. was actively controlling Krum’s every action, it was probably Crouch’s emotions being used to cast the Cruciatus Curse, as the Dark Lord says.
Also, people can do things under Imperio they normally can’t.
People placed under the Imperius Curse can be capable of things they wouldn’t be while not under it. “Moody” commands Neville to do gymnastics that were clearly beyond his usual capabilities.
“Moody began to beckon students forwards in turn and put the Imperius Curse upon them. Harry watched as, one by one, his classmates did the most extraordinary things under its influence. Dean Thomas hopped three times around the room, singing the national anthem. Lavender Brown imitated a squirrel. Neville performed a series of quite astonishing gymnastics he would certainly not have been capable of in his normal state.”
- Harry Potter and the Goblet of Fire, Chapter 15 (Beauxbatons and Durmstrang)
If Krum was commanded to cast the Cruciatus Curse, it might outweigh the fact that he doesn’t “mean it”. His Cruciatus under the Imperius Curse probably wouldn’t be as effective as one cast by someone like Bellatrix while she’s actively enjoying it, but it would probably be effective enough to disable or slow down the other champions.