He was only returned after the Swedish embassy learned about his condition. He had already been in a coma for a year by then and would have stayed in the DPRK for the remainder of his sentence otherwise.
If they claimed that a man in his twenties had died of a heart attack as they did with Japanese citizens they abducted, that would have been an even bigger scandal.
But the way he was taken care of and returned (they could have claimed he died) seems to suggest that he was not put into this condition on purpose.