The ghost wants the Gods to be the judge of her actions. Another reason is that the ghost thinks that she is weak and is not capable to win the fight. Lastly, maybe the ghost, her husband, do not want to harm her.
Though Hamlet speaks harshly about Gertrude , he seems to have a soft corner for her Maybe the ghost is still in love' with his wife and doesn't want anything bad to happen to her.The ghost considers Gertrude as a weak person and so does not wish to harm her.
