Either Thresh Wasn't Evil From the Start...
Or the other monks of the Blessed Isles were dumb as rocks and it's a miracle they lasted as long as they did.
If Thresh_ wasn't_ a sadist from the beginning, then we've got a pretty reasonable fall-from-grace story in which no one did anything terribly stupid. Monks assign a guy with a strong will to look after all the spooky artifacts, spooky artifacts end up being spooky enough to turn Thresh evil, Ruination makes it all worse, and so on.
But if Thresh was a sadist from the beginning...then who's the brainiac who stuck a known sadist in a vault full of powerful magical relics unsupervised? That decision would require an unprecedented level of stupid. For us to accept that premise, we would also have to accept that a society of enlightened scholars were somehow simultaneously total morons.
Heck, even if Thresh was a sadist and just really good at hiding it, the other monks would still be idiots for having such a poor vetting process for an extremely dangerous and easily abused position of power.
If that many people were that stupid, the Ruination would be redundant because the Isles would have already mismanaged themselves into oblivion.
Therefore, I find myself assuming Thresh was at least morally neutral before the artifacts and Ruination because it's the only situation in which I don't also have to assume the island paradise of enlightened scholars was run by smooth-brained cavemen.