It just doesn't make sense to me. From the dialogue in ICC, we're suppose to think Arthas is trying to break Bolvar to serve him.
But why is that even necessary?
We have a long list of victims fallen to the Scourge, and was made into a slave of Lich King against their will. We have Sir Zeliek who was clearly not broken, yet cannot resist the Lich King's control, which is the entire point of the Scourge in first place: Freewill doesn't matter, you will serve regardless.
So why is Arthas indulging in some senseless torture that serves no purpose? Or is the "Breaking" a necessary part of Scourge conversion that all of Lich King's minion have to go through? Is Blizzard trying to tell us Bolvar's will is that strong?