At the end of "The Lagoon", what does Arsat plan to do?Bury Diamelen, seek revenge, or forgive the killers?
Of the three choices given, "to seek revenge" is the closest to the plan of Arsat's. Arsat's return is more of an act of atoning for the guilt that he has always felt for having cost his brother his life than one purely of revenge. He will risk his life—"I am going back now"—but there is no one for whom he is committing such an act that would be brave were Diamelen still alive.
The choice now becomes irrelevant, for his return can be seen not as the pursuit of lost honor and bravery but as an expiation of guilt, an easing of the conscience, even a form of suicide—the ultimate cowardice.
Joseph Conrad's narrative has an existential theme to it. When Arsat finishes telling the white man his plan to return and confront his enemies, he observes, "I can see nothing," and the white man replies, "There is nothing." The two men's observations suggest the concept of the absurdity of life. "The Absurd" in existential philosophy refers to the struggle of humans to seek meaning and inherent value in life when they are unable to find any. Instead, man is as Arsat is—standing alone, "look[ing] into the darkness of a world of illusions."
At the end of the story, Arsat plans to seek revenge. The white man Tuan is leaving the lagoon and urges Arsat to come with him, but Arsat refuses. Arsat's forbidden lover Diamelen has died, and Arsat feels that there is nothing left in the world for him. He resolves to return among his enemies and fight them on behalf of his brother, who is also dead. Arsat's brother died trying to hold off angry tribesmen who were sent to bring Diamelen back to their noble leader, whose concubine she was. Arsat feels responsible for his brother's death, and must bear the guilt for deserting him so that he could enjoy a new life with Diamelen.
Despite his resolve to earn a measure of redemption for his past weaknesses, Arsat's decision is devoid of meaning because he has already lost everything. In order to pursue love, which in actuality is a noble objective, he has had to act with cowardice and treachery towards his own brother, and now both his lover and his brother are gone. Arsat tells Tuan that "he can see nothing" in a world where there is no purity of motives, nor clarity between what is right and what is not.