In order to judge Aquinas for believing that it is just for people to go to hell, I must first understand what he believes about suffering.

As far as I can tell, everyone agrees that Aquinas believed that ALL forms of badness are merely privations of the good. So badness could not have being. When Aquinas talks of badness, he is just referring to some concept of non-being, the mere lack of goodness.

Therefore, when you go to Aquinas’s version of hell, you do not experience any badness (from my perspective), because that has no being in itself.

Personally, I have no problem with any world where no one experiences badness in itself. Experiencing a minimal goodness and thinking about how you are being deprived of greater goodness is still good in itself.

The only way that the thought of being deprived of higher goods could be bad is if that thought caused intrinsically bad feelings.

“Wait, you mean that when someone burns and suffers from burning, Aquinas thought they are merely being deprived of goodness?”

Yes. https://en.wikipedia.org/wiki/Absence_of_good

The mind of a being in hell has an experience that it calls suffering, and the mind continually labels this suffering “bad,” as in “less good” or “not preferred.” But the labeling of something as bad, less good, or not preferred is not a bad event in itself, so there is no real problem.