The exhausted ferocity of Decoud's letter gives Conrad a wonderful way to present
complex notions via the language of apparently careless fatigue. What Decoud is
really saying here goes like this: Charles Gould is actually an honest man "behind
his taciturnity," i.e., in his subjective soul. But his honesty is nevertheless
"stained." Why? Apparently because he has gotten "hold of the substance,"
i.e., acted in the physical world. Compare Mrs Gould's later line, "There
was something inherent in the necessities of successful action which carried with
it the moral degradation of the idea."
The message seems to be this: in the inner world the soul may remain subjectively
"honest," but action takes place in the external world, and causes real
consequences that must be held against you as a stain and moral degradation. In
Charles Gould's case these include an alienated wife and a civil war.