I have said it before, but I don't believe that for a moment. Like you said, it makes no sense Harold would kill himself when the whole idea was to avoid death flags. Plus he was so powerful to begin with, and would naturally grow more powerful growing up, that why would he even need the vitality-eating sword, except for very special circumstances? It was actually smart to use it once in the presence of the girls, so that if Justus talked with them, they would confirm that his using it, making Justus happy.
Of course the worse alternative is that the story that became witless and Harold has actually been killing himself with the sword. It would then be up to Leafa and Erica to somehow restore his vitality. I really hope this isn't the case.
The previous chapter already seemed to reveal Justus, at this point, isn't even the main active villain. He ought to grow into the biggest evil later, but perhaps the other villains play a much bigger role than the simple game plot would have indicated. Like the army general. So, Harold might still be in the process of trying to figure out how much needs to be done and how many people are involved. Besides, I don't believe Justus is weak. He should have his own, solid defences. Not to mention the whole kingdom would try to save him, should something happen.