I'm not saying Dongtae is grey. He means well, but he is naïve. He was decent enough to refuse any quests that involved harming other people: I can respect him for that. He understood it very early on, when he felt bad for being told to tell a minor lie in exchange of Dice in chapter eight. His initiative of creating the guild to restrain Dicers' worst instincts was also well-intentioned. And we have just seen how much of a spectacular failure that plan was.
A person can be good and still be used as an instrument to evil if they are not wise. Dongtae had every right to think that improving himself the best way he could and escaping that horrible life was justifiable. However, he never
thought about the implications of the quest system. He never gave it careful consideration. He went to the extent of telling Taebin "why should we contain them? Isn't it good that they are improving themselves and getting happier?". He said that when it was already obvious that people were getting hurt in the process.
I don't believe the author's thesis is that "everyone has their own way of changing and living their life and rightfully so". His point is precisely that not all ways of changing are valid:
one must not harm another to get what one wants. The Dice and the quests are the precise opposite of that, because X is the one in control and he does
not mean well. He is deliberately making people harm each other. And while Dongtae may not take part in that, he shares responsibility to the extent that he uses Dice.
Especially because he is not able to tell when X is manipulating him. For all we know, X can give Dongtae an innocent-looking task and Dongtae, on carrying it out, may do a lot of harm without knowing until it is too late.
In other words, even after all that has happened, Dongtae still refuses to abandon Dice. I believe he will only realise his mistake when something really terrible happens.
My point is that Eunju knew about that all along. I don't really care about her state of mind, her role in the story is of someone who sees below the surface. Her decision and Dongtae's, as far as Dice are regarded, may be
morally equivalent - both decided to go about it in ways that did not corrupt them; but they are
intellectually different, and Eunju's is superior, because she
thought about the consequences of the quest system. So both Dongtae and Eunju are good, but Eunju is also wise (although I believe Dongtae is much better now and will eventually reach her level if he tries hard

).
However, to be honest, Dongtae has two things that excuse him (in part):
i. He became a Dicer by accident.
ii. For a long time, he was the only Dicer around and he probably thought that, so long as he refused morally objectionable quests, he would be fine. In that sense, he can be forgiven because he did not have to think about implications, since the outcome depended only on his decisions and not those of other people. When Eunju learned about Dice much later, she had better evidence to reflect on. She knew by then that the quests led people slowly towards wickedness, and that's when she said "if that's so, I don't think these things should exist". She had a much better panorama of the situation than Dongtae when she made her decision. But if you ask me, if she had been offered Dice under the same circumstances as Dongtae, she would still have refused to use them.
But I've written too much about that. We had better agree to disagree on the subject.