The irony of Camus' "The Guest" is that while Daru the teacher is an honest and fair-minded man, and his "guest," the Arab prisoner is a murderer, both men find themselves facing similar fates.
When Balducci (the policeman) arrives at Daru's home, he tells Daru that he must assume the responsibility of the Arab prisoner, taking him on to the jail as Balducci has too much to do. Daru refuses, but Balducci leaves the Arab prisoner with him anyway.
In a short time, Daru becomes convinced that he cannot do as he has been asked. He takes the Arab prisoner part of the way to the jail, but stops at a point in the road where they must choose to go toward the jail or away from it. Making this clear to the Arab, Daru turns and starts his journey home. At a short distance away, Daru turns around and sees, amazingly, that the Arab man is walking in the direction of the jail. The man has seen the inevitability of facing the crime he has committed and goes, knowing he will punished for taking another man's life.
Daru is as much a prisoner as the Arab. He is told he must take the Arab to jail. He believes that he has a choice: to do so, or to let the Arab decide. By refusing to decide the Arab's fate one way or another, Daru has still made a choice. However, when he returns home, he finds that he, too, will be punished for what he has done. However, it will not be for a crime he has committed, though those who threaten his life believe he has; because they are certain Daru has taken the Arab to jail, the Arab's friends promise to return and kill him.
...on the blackboard...sprawled the clumsily chalked-up words..."You handed over our brother. You will pay for this."
The irony is that the Arab is being punished because he actually did something wrong. Daru will be punished though he has done nothing wrong. In truth, as soon as Balducci brings the Arab to him, unless Daru lets the man go while they are still at his home, he will be blamed for anything that happens to him after they leave Daru's house.