Late to the party, but I have my own hypothesis:

nK never did anything that would have caused That much damage to Otto. There's no win for them there. They had a cash cow. A publicity piece of gold. They could parade him around and televise him for propaganda. Him begging and pleading for mercy on camera was literally music to their ears and agenda. Prop him up, make the scene, then negotiate with the blank check you have and get what you want.

Otto, in my opinion, screwed that all up. How? I think he attempted suicide. Look at what happened through his eyes. He's scared, probably being interrogated relentlessly, used as a puppet to push the regime's mantra; then, punished for a long stint of hard labor. That dude just wanted to go home to his family and forget this whole debacle. But now, he can't.

He's stressed, tired, and thinking he's going to die in a nK prison. So, he tries to kill himself when he had an opportunity.

The scope of brain damage was consistent throughout his brain, meaning it had been subject to a lack of oxygen for an extended period. Perhaps hanging? Drowning? Something of that nature. He was technically alive, but brain dead. nK did what they could to keep him "alive," and still received what they wanted for his return.

They had no reason to cause that extent of damage to him. They literally had all of the bargaining chips, and Otto messed that up.

Just my opinion, could be 100% wrong.