Conrad's masterful descriptions of the sea and the situations it creates achieve an unusual intensity and tension. With the stowaway Leggatt an essential theme of Conrad's is intensely examined: Man is not an alien solely by himself: he shares being an alien with every other man.
As a sailor on the ship Sephora, Leggatt has killed a man who would not obey orders during a storm, endangering the other sailors.
After he is allowed on board when discovered clinging to the captain's ship, Leggatt is described as
His expression was concentrated, meditative, under the inspecting light...such as a man thinking hard in solitude might wear.
Leggatt's description of the sailor whom he killed is reflective of his irrationality:
It's clear that I meant business, because I was holding him the throat still when they picked us up. He was black in the face. It was too much for them....They had rather a job to separate us, I've been told. A sufficiently fierce story to make an old judge and a respectable jury sit up a bit. The first thing I heard when I came to myself was the maddening howling of that endless gale, and on that the voice of the old man.
Leggatt does not appear to be evil, for when he relates his history to the captain, he explains that the captain of the Sephora had him locked in every night and the men were afraid of him. He tells the captain,
You'll say I might have chucked him aside and bolted out, there and then--it was dark already. Well, no. And for the same reason I wouldn't think of trying to smash the door. There would have been a rush to stop me at the noise, and I did not mean to get into a confounded scrimmage. Somebody else might have got killed--for I would not have broken out only to get chucked back, and I did not want any more of that work.
The only suggestion of evil seems the mention of Leggatt of having "the brand of Cain" upon him.