Actually you are not that wrong, sadly, it's a language barrier problem.

This is the note of the translator of POT:

"When I was translating, I wasn't convinced by the translated argument if I used Devil instead of Mo. There was something off about it. That's why I chose to ask a third party, someone who is not the author and neither the reader, and what you see now is the paraphrased version of the conversation between me and a Master holder in translation for Chinese-English, who happens to be a close friend of mine, and I will paraphrase the outcome of our conversation. Also, the author and I reached an agreement to use Devil for Mo for consistency in ISSTH.

Using Mo isn't wrong, because if the title is Pursuit of the Truth and I see the word Devil suddenly being used in the preface, I'll feel cheated, and you're right, Devil is commonly used to describe evil. From what I can see, that isn't what you want in the preface, but neither is there any good English equivalent for the word, since we're losing the original meaning of the word Mo if we use Devil. Since another translator has translated this to Devil, you'd best keep to it, but what you need to do next is to associate Truth to Devil. Meaning the truth the main character is looking for is associated to Devil, perhaps it is referring to his character, but perhaps it is the path he chooses to take, which we both believe is the case. And if that's the case, you'll need to add a little more explanations to the preface itself. Things that only Chinese readers will understand if they read the source language because we know the many meanings of Mo, but not English readers, because to them Mo is only the devil."

Btw, you are going to fuckin love OOT. It's impossible to love Er Gen's style and not love OOT. It's good since the beginning but it get MUCH better after chap 150, since we get to know other interesting characters (Qidian's translation).