What event in Act V suggests that Macbeth is so exhausted by murder and bloodshed that he is almost used to the pain of death?

The correct answer is - The news that lady Macbeth has killed herself.

Even though Lady Macbeth made her husband kill the king, it seemed that she was plagued by guilt more than the indecisive Macbeth. He grew accustomed to murder in order to achieve his goals, whereas his wife slowly started descending into madness. Ultimately, she killed herself, and her husband wasn't even surprised - he became desensitized to death.

When Macbeth heard that the trees of Birnam Wood are headed to Dunsinane, it is then that he declared that he is weary of the sun and wish that the state of the world can be undone; however, he will continue to fight and he will die fighting. This lamentation unfolded in Act 5 Scene 5.