What do apparitions usually portend according to the witnesses who have seen the ghost in Act 1 of "Hamlet"?

The men on guard have seen the ghost of the dead king Hamlet walking about at night.  They are afraid because ghosts are usually considered as evil spirits and negative omens of things to come.  As it turns out, they were right.  If Hamlet had never spoken to the ghost, he would have lived in ignorance of his father's murder.  Although he would probably have still hated his uncle and been angry with his mother for their o'er hasty marriage, Hamlet would not have been bent on revenge.  This might have spared Polonius, Ophelia, Laertes, Claudius, Gertrude, Rosencrantz, Gildenstern, and even Hamlet himself.