How Can Eternal Damnation Be Justified?

People live for a finite period of time.  No matter how badly they behave during their lifetime, how can infinite punishment be deemed appropriate by anyone but a raging, hate-filled entity?  Take Adolf Hitler for example.  He was a bad guy, responsible for the deaths of millions.  If anybody deserves some serious retribution, it's him.  But think about it: if he had to spend a century in the torturous lake of fire described in the Bible for each individual who died because of his actions, that would come out to somewhere around a billion years of agony.  And yet, as measured against eternity, his punishment would have only begun.  It doesn't sound like justice to me.  It sounds like revenge on a monumental scale, and I can't fathom the level of malevolence that represents.  Could a loving God really be that cruel?  So how about us regular sinners?  We get the same punishment as Hitler?  It just doesnt seem reasonable.