The Horrifying Oversight Revealed in Thresh's New Bio
First off, let me make this clear: I'm talking about an in-universe oversight. Nothing to do with the real-life people who wrote it.
Now that's out of the way, let's get into the lore. Thresh's order had a guy with obvious personal problems who was known for being a jerk. Instead of helping him find a way to be less of a jerk, this enlightened civilization of brilliant scholars basically stuck him in a basement with spooky stuff.
In an utterly shocking turn, this move did not instill Thresh with a strong sense of gratitude or loyalty. What were they thinking? Did they believe evil was an inherent trait of a person rather than something that could be fixed? Was it common practice on the Blessed Isles to isolate "difficult" people and let them get worse out of everyone's sight? If so, how many more people very similar to Thresh had the same reaction he did when the Ruination came?
These people were supposed to be smart, but they goofed big time here. No one is born evil. Thresh's monstrosity could have been prevented if anyone put in a smidgen of effort, but they took the easy way out and stuck him in a magic hole in the ground.