How was Hamlet's insanity caused by the ghost's evil intent?
First, I'm not sure that Hamlet was truly insane. He did a good job of pretending he was mad, but as he tells his mother, he is pretending to be mad so he can find out the truth about his father's death. Secondly, you assume the ghost's intent was evil. That is a question that has been debated for centuries. The Elizabethan audience for which Shakespeare wrote would certainly have believed the ghost was evil. If you look at the ultimate consequences of the ghost's remarks, it certainly looks as if he wanted harm to come to Hamlet's family. However, much of the evil comes from the characters themselves so critics have debated if Shakespeare, himself, wanted the ghost to be seen as evil. However, the short answer to your question is that Hamlet's insanity was not caused by the ghost because Hamlet was never truly insane and the question of the ghost's intent can be endlessly debated. It would be better to ask 1. Is Hamlet really insane? and 2. Is the ghost's intent really evil? Those questions have been examined by enotes before and links to the answers are below.