I think my problem is that I underestimate the sins, this makes me think that hell is too much of a punishment, and it makes me not believe that anyone deserves it. I understand that the punishment is eternal because their intentions or arrogance are eternal, but since the punishment is eternal, why is it that painful? I don't understand what these people could've done to deserve such a fate, but I know Allah (SWT) is All-Just, so he wouldn't punish them more than they deserve, so how do I come to understand that this is what they deserve?

These people violated the King of the Universe. The violated their creator who gave them life and gave them every cell in their body. They used his resources to disobey him and spread corruption. And they did that all knowingly and deliberately. Yes they deserve painful punishment for their defiance. Allah may still choose to forgive them, but they deserve that punishment. They caused pain to so many people by not letting the law of God be implemented. They helped tyrants torture so many people with painful punishments. They deserve a painful punishment.