Because for one to truly believe and feel that one is evil and worthless, to feel that one is truly morally deserving of eternal torture - that necessitates self-hatred.
Consider: You hear in the news that a man (let's call him Otis) pretended to be a loving partner, so that he could get a woman to trust him. Then used that trust to give him an opportunity to molest and kill her children, and show them to her, gloating over her foolishness while smiling over what he'd done. Later he shows pride rather than remorse for his actions.
I'd consider "hatred" to be a reasonable reaction to Otis. From a retributive moral standpoint, he deserves unspeakable things to be done to him. He deserves no respect as a human being anymore. He is evil.
And I don't already hold that he's evil just for being human, like a Christian would.
Now, what would it take to get him to deserve eternal punishment? To be considered even more evil than I'd consider him to be? Whatever it took - whatever he'd have to do in order to earn it - would that increase, or decrease, the amount of hatred directed at him? Increase
, if anything, right?
Well, let's transfer that to someone else, from a Christian perspective. Let's call him Scott. Scott's just a normal guy. He deserves not just unspeakable punishment, like Otis does from my perspective (which doesn't include the baseline of human evil that Christianity includes), but eternal, ultimate
punishment. He is worse
to a Christian than Otis is to me (and Otis would be unspeakably bad to me, earning hatred and all). For a Christian to genuinely feel those things about Scott, must entail them hating Scott. Hating him more than I would hate Otis.
Now, Scott is another person. But he's just a guy. He's not really different from us, from a Christian perspective. We are just as corrupt. We deserve eternal torture as much as he does. We deserve punishment to a greater degree than I'd hold Otis to deserve punishment, from a Christian perspective. Our self-respect as humans is lower, from a Christian perspective, than my respect for Otis as a human would be. We have to hate ourselves - if we seriously feel that way.
Fortunately, Christians don't seem to seriously feel that way. Unfortunately, they say they do.