Why had Bod been warned to stay away from the far corner of the graveyard in The Graveyard Book?

Bod has been warned to stay away from the far corner of the graveyard known as Potter's Field, as that's where the criminals and suicides are buried. This doesn't mean that the people who are buried there are necessarily bad; it's just that, as Silas tells Bod, this is unholy land—land that has not been blessed by the church.
Once upon a time, it was common for the graves of suicides, criminals, and actors (yes, really) not to be consecrated by the church. This is because such people were regarded as unrepentant sinners. There was also an element of social snobbery involved, as many of the poorest were buried in unconsecrated ground due to their not having enough money for a proper funeral. It is largely the ghosts of such poor unfortunates that haunt the far corner of the graveyard and whom Bod must avoid.