Infinite punishment for finite actions is, itself, infinitely unjust- no matter the severity of the act.

There are things people do in their ~80 years of life that could arguably deserve hundreds or even thousands of years of punishment. But millions of years? Billions? The proportion of crime to punishment goes to 0 with an infinity in that denominator. It's inevitably incomprehensibly excessive.

There is a distinction between "deserve" and "should".