Maybe the ghost was a figment of Hamlet's imagination, or maybe the sin of her hasty wedding meant that she couldn't see.
Hamlet's hesitation is his tragic flaw, which he pays for with his life. When he sees his uncle praying and fails to kill him on account of the fact that he will go to heaven if he dies when at prayer was wrong- he would gone to hell as he was still enjoying the proceeds of his murder (the queen and the kingdom). This is Hamlet's flaw and the reason why he has to die. In tragedy, the hero has a flaw that is the cause of their death.
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