Yudhishtira was simply In hell. He was not Tortured in hell.
The keyword is 'behold'.
A king is supposed to know the sufferings of their subjects. The only way they can do so is to experience pain themselves. But because they are kings, they can't be punished by anyone while on earth. So they are shown a sample of tortures in hell after they leave the mortal coil.
Suppose you are being chased by a tiger, and you fall into a pit and break your leg. You are screaming in pain, but you can't hear your own voice. That's because you are in a nightmare and realize it as soon as you wake up.
Now, was the tiger real ? No. But was the fear/suffering real ? Yes.
So, you get to know suffering without actually being punished. Same thing for kings (if they were just). But if they were unjust, they would still have to actually suffer.
Now, the 4 Pandavas actually had done some mistakes, hence they actually had to go to Hell for a miniscule amount of time.
Also, Yudhishtira had requested Indra/Yama that - 'Take me to where my brothers went' (after they had fallen before him while walking on the path to svarga), he didn't say 'Bring my brothers to where I am'.
So, for 3 reasons - king must know pain of subjects (atleast through illusion), 4 pandavas actually had minor sins, Yudhishtir wanted to see/be with them - this natak was done.
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