“You'll be glad, too, when the end comes.” When and where was this said in And Then There Were None?

This line is delivered by General Macarthur and it occurs in chapter 8, after the first day has passed. Earlier in the chapter it is clarified that Vera felt restless the entire morning, so this scene likely takes place in the afternoon of their second day on the island. 

"After a while Vera strolled slowly down to the sea. She walked along towards the extreme end of the island where an old man sat staring out to the horizon."

The old man she joins is General Macarthur. The description of the setting is minimal, but it is implied that Vera and the general are sitting near the water. 

The line "You'll be glad, too" is the General referring to the relief that he mentioned a handful of lines earlier. He is convinced that they will all die on the island and has come to the realization that he feels relieved to know that he will soon be free of the guilt he has been carrying with him for so long. He tells Vera that when her turn to die comes, she will also feel relieved.