General Macarthur comes across as a bit of a doddering fool. He’s always sitting off by himself and mumbling about cryptic things. But the truth is that Macarthur is a very tired and lonely old man, and that he’s been plagued by what he did for too long.
So if he’s nothing but an old, remorseful man, why does he have to die? Justice. Justice Wargrave, to be precise. He sent his wife’s lover into battle to die, which is totally against the man code, or something. Still, we know that Macarthur is about as good as a criminal gets, because he’s knocked off so early: he’s the third character to die, and he gets hit in the back of the head. If you have to get murdered on an isolated island, that’s not a terrible way to go.
Macarthur’s guilt has isolated him from his friends and neighbors, especially after his wife dies. Once he’s on the island, he’s not even sure he wants to leave: "Funny, just this minute he didn’t want much to get away from the island… To go back to the mainland, back to his little house, back to all the troubles and worries." (5.128)
This tortured and guilty past makes him relatively complacent about waiting for death. Once his fellow guests start dying gruesomely, General Macarthur just chills out and waits for his turn. Like Vera, he ends up welcoming death as a reprieve from guilt and suffering: “He knew, suddenly, that he didn’t want to leave the island” (5.131).
He doesn’t mind staying there forever at all.