Do you believe in karma?
Sin unto death is the performance of a sin that is so vile it guarantees a spiritual or physical (and maybe even both) death.
It’s common knowledge that murder is a terrible sin — something that wouldn’t be easily forgiven without an efficient amount of repentance and regret, in addition to never committing the same act ever again. Mr. Harvey fails to meet the criteria to save his tarnished soul. It’s true that he attempts to quench his need to kill humans by killing lesser beings (such as animals) but that doesn’t change the fact that he keeps on. Presumably, he’s never asked for forgiveness and even if he did, it wouldn’t work as he doesn’t stop his murders anyway.
You can say that Mr. Harvey had it coming. There’s a sense of foreshadowing in Susie’s earlier statement on how an icicle would be the perfect murder weapon. True enough, that’s exactly how the man dies — caught in the middle of trapping his new object of torture before an icicle moves to cause his death. Finally, his previous sins had caught up to him and gave him a taste of his own medicine. Although his death isn’t as brutal and disturbing as how he’s murdered those other girls, it still stands that he is killed unaware and, hopefully, is sent straight to burn in hell.
According to Hamlet, those who die without seeking forgiveness has nothing but hell to welcome their souls in the after-life. However, after Sebold’s alteration of the supposedly simple idea of heaven, it remains suspicious as to how she would have dealt with Mr. Harvey’s soul had she went into detail about it. Unlike Susie and the other murdered girls, the murderer’s spirit would certainly not be welcome in the in between.
Maybe Sebold doesn’t even have an alternative to hell. Maybe it really is the only place where a soul like Mr. Harvey’s can spend eternal life in.