Why did the Man Jack leave the graveyard without the child in The Graveyard Book?

The Man Jack has just committed a brutal murder: he's killed all members of Bod's family. Bod himself, who's just a toddler, has fortunately managed to escape this scene of terrible carnage and toddles his way up the hill to the nearby graveyard, with the Man Jack hot on his heels. When he gets there, the Man Jack looks high and low for the child but cannot find him.
A strange figure by the name of Silas hides Bod and convinces the Man Jack that there are no babies in the graveyard. The evil killer turns on his heels and makes his way back down the hill and disappears. Silas's intervention has undoubtedly saved Bod's life. He will go on to be Bod's guardian, protecting him as best he can against all manners of evil.