The ending of the story is shown to place Arsat in a very difficult position from which there is no escape. Arsat, when he fled with Diamelen, had to choose between going back and helping his brother fight the enemy or his love. His choice to run away with Diamelen is therefore an act of moral cowardice, because he failed to act like a warrior and a man. It is only at the end of the story, when his beloved Diamelen dies, that Arsat tries to redeem himself with his intention of returning to his enemies and fighting for the death of his brother. However, this is too late. Now Diamelen has died, Arsat has no massive decision to make as he has lost everything that is dear to him. His desire to avenge the death of his brother can therefore be seen as nothing more than an expiation to relieve his guilt through a form of suicide.
This is why Arsat is described as being afflicted by a kind of paralysis as he stares "into the darkness of a world of illusions." Conrad is here commenting on the impossibility of our ability as humans to act in a pure fashion without differing emotions or choices where whatever decision we make we are morally culpable. Note the way that the white man tells Arsat "there is nothing." Arsat and the white man are left with nothing more than uncertainty, illusion and the image of the lagoon that is opaque and impenetrable in its darkness.