In Act I, Scene 5, Hamlet said, "It is an honest ghost." Yet in this scene, he says, "The spirit that I have seen may be the devil." What has evidently happened as Hamlet has thought about the strange events?

He had started to doubt himself, unsure of how and why the ghost had appeared and for what purpose.

Explanation:

Act I of William Shakespeare's tragic play "Hamlet" shows the young prince Hamlet meeting his dead father's ghost for the first time. And then came the revelation by the former King's ghost of how he had been murdered. This revelation took Hamlet by surprise but also made him vow to exact revenge on the culprit.

When Hamlet said "it is an honest ghost", he was fully sure of what he had been told by the ghost. But later on, he again said "The spirit that I have seen may be the devil", implying that he's starting to question the whole situation. Earlier, he had been so much consumed with grief about his father's death that when the ghost came, he was fully co-operative with the plan and the story. But later on, when he isn't with the ghost and had time to think more clearly, he began to doubt his own decision.