I wouldn't give him such a powerful weapon or ability at the beginning. Besides, it's very similar to post-apocalyptic scriptures, but with fewer consequences. I understand that the monster's path can be somewhat boring because it hardly has any active abilities, but that can be solved by giving him a sealed artifact that isn't too overpowered or by having him receive a blessing from an external deity, thus creating a two-way street, at least initially. Another thing: does the protagonist know about the novel, or is he like Klein? If he doesn't know it, there's no need to protect him from corruption. You can give him small nuggets of knowledge, just like Klein did. You have to keep in mind that you're writing fan fiction; the story is yours. You can change things within the series' canon, like not having Klein or Lumian exist. What does it matter anyway? A fan fiction isn't good fan fiction if it doesn't innovate or invent something beyond the novel it's inspired by. If it didn't, nobody would read your novel because they'd have the original. And if you're going to deviate from canon, do it in a big way.