It is fact that the revelation of Ghost is supposed to be bad omen.Here we see the father of Hamlet as ghost after death comes to disclose the secret of his death.The disclosure obsessess Hamlet`s mind and he resolutes to take avenge.He takes revenge in which his life is too lost.so the ghost when ever appears it creates problem for mankind.Simultaneously he creates problem and Hamlet loses his life.It means ghosts are nothing else but evil spirits.

I see the ghost, even though other people can see him, as something from Hamlet's imagination. The ghost is really only there to get the plot in motion, and since ghosts don't exist, I think it's all in Hamlet's mind.

In my opinion, he isn't an evil spirit, more like a spirit in distress. He's burning in purgatory for god's sake, that's the whole purpose of the story, to kill Claudius and put his father's soul to rest. It's kinda like what the greeks believe, in the Illiad, Achillies is haunted by patroclus's ghost, Achillies is so heart broken by his death, he doesn't burn his body so Patrolcus spirit is just walking around and he begs Achillies to burn his body so his soul can rest.