how do Daru's feelings about the plateau shift from beginning to the end of the story?

The end of The Guest embodies the overwhelming solitude that pervades the narrative. The landscape symbolizes Daru's isolation. However, where he once felt "at home" on the plateau -- because he chose to live there, thus embracing its absurdity -- the episode with the Arab has left him in a state of radical uncertainty. He views the Arab moving through the harsh landscape with new eyes, feeling it's desperate solitude. He no longer feels "at home" on the plateau, because the natural landscape has become associated with the moral dilemma of the Arab, and with the Arab's choice to walk toward imprisonment. In failing to make a decision, he has allowed the Arab to impose a new meaning on the landscape.

Daru's return home emphasizes this newfound despair. His window looks south, which is the direction the Arab would have taken for freedom. Thus the window reminds him both of the Arab's failure to choose freedom, and his own failure to choose the Arab's fate.

Daru's alientation from the plateau region becomes all the more concrete, and all the more dangerous, with the writing on the blackboard. Camus leaves it unclear who wrote the note. Perhaps Daru himself wrote it as an expression of his morally complex state of despair. However, the note may have been written by Daru's friends. If so, they replay the moral problem that faced Daru. Though the people who wrote it have no access to Daru's moral struggle, they are in a position to pass judgement upon him for it. We assume that they will not be as wishy-washy as Daru was.