Why does Hamlet say the ghost is the devil? What makes him say that?

When Hamlet says, "the devil hath power to assume a pleasing shape", he is interpreting a verse from the Bible that says "for even Satan disguises himself as an angel of light."( II Corinthians 11:14). During Elizabethan times, ghosts were thought of as evil spirits. This belief is echoed by Horatio when he warns Hamlet not to follow the ghost at the beginning of the play. He says:

What if it[ the ghost] tempt you toward the flood, my lord,(75)

Or to the dreadful summit of the cliff

That beetles o'er his base into the sea,

And there assume some other horrible form,

Which might deprive your sovereignty of reason

And draw you into madness?

Act I, Scene IV, Lines 75-80

Thus Hamlet calls the ghost the devil because that was a common belief at the time.