Tbh, I think it was pretty obvious Arthas was going to become a DK, and Jaina ought to have seen the signs: on the way to Strathholme he basically aggroed everything in his path, made her pick up his slack continuously without checking how she was doing on health or mana, until she was completely exhausted— and then when finally she asked for a mana break, he was impatiently telling her to hurry up. If that’s not DK personality, I don’t know what is.