The man Jack, the murderous brute who wiped out Bod's entire family—but who thankfully didn't manage to get his hands on Bod himself—is a member of an ancient, mysterious order called the Jacks of All Trades.

Back in ancient Egypt, it was prophesied that a child would one day be born who would walk the boundaries between the living and the dead. It was further prophesied that if the child in question grew to adulthood, then that would be the end of the order and everything it stood for.

The Jacks sent what they thought was the best of them to kill Bod in order to protect their supernatural powers and preserve their order. Unfortunately for them, the man Jack was unable to carry out his grisly mission, and Bod was able to escape his evil clutches and head on out to the graveyard.

Even so, Bod remains in considerable danger. The man Jack, now going under the guise of Jack Frost, has unfinished business and is determined to kill Bod if it's the last thing he ever does.

Thankfully, he fails in his mission, even though he has four other Jacks in tow to help him. At the graveyard, three Jacks are destroyed by Silas, and the man Jack, the one who killed Bod's family, is himself killed by the Sleer, who drags Jack into his lair.

In The Graveyard Book, why does Jack want 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

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.”