The questions we must first ask in relation to our question:
What distinction does the bible make regarding judgement?
If all forms of judgement are immoral, does that make God immoral?
When would it be right for us to judge someone?
The idea of an eternity of damnation for a few years of sin seems morally incoherent; however, we must first consider the holiness and authority God holds.
Hypothetically, if someone were to assault a regular member of the public, the law may let them go free with a simple apology or community service. Consider the same scenario, but instead of a member of the public being assaulted, it is the king or queen of the country. The assault would be met with far more severity and outrage.
How much more serious, then, would a lifelong assault on an unfathomably greater being be?
The idea of our rebellion against God is not just bound to our life on earth, however. If it’s the case that we keep sinning in hell, then we would be eternally indebted for our eternal wrongdoings, henceforth deserving of an eternal punishment, just as a prisoner who keeps committing crimes would keep getting their sentence extended indefinitely.
The question of whether someone can get out of hell by seeking forgiveness is not overtly answered in the Bible, but most Christians would agree that once you’re in hell, that’s it. Scripture certain implies this. This gives birth to a much more proactive question, that being:
Is your dismissal of God purely because of your disbelief, or is it because you are not willing to submit to His authority?
Conclusion
It seems fair that sinning against an eternally great being can result in an eternally great punishment, regardless of whether we keep sinning after death or not. If hell is separation from God after death, why would someone who chooses to live their current life without God be surprised to be apart from Him for eternity?