In Neil Gaiman's novel for young readers titled The Graveyard Book, why does Jack need Bod dead?

In Neil Gaiman’s novel The Graveyard
Book, a man named Jack succeeds in killing all the members of a particular
family except the young infant he particularly wants to murder.  This infant, eventually
known as Bod, happens to be absent from the scene of the crime when the killings occur.
He is adopted and raised by the residents of the local graveyard, and only many years
later, in Chapter 7, does he discover why the man named Jack – and indeed a group of
men, all named Jack – want him dead.

In Chapter 7, one of
the “jacks of all trades” reveals to Bod that

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Long time ago, one of our people – this was back
in Egypt, pyramid days – he foresaw that one day there would be a child born who would
walk the borderland between the living and the dead. That if this child grew to
adulthood it would mean the end of our order and all we stand for. . . . And we sent
what we thought was the best and the sharpest and the most dangerous of all the Jacks to
deal with you. To do it properly, so we could take all the bad Juju and make it work for
us instead, and keep everything tickety-boo for another five thousand years. Only he
didn’t.

The Jacks, therefore,
tried to kill Bod to prevent the loss of their vast supernatural powers and to protect
their strange and ancient “order.”