The key word is "tried"; although Decoud is given an almost omniscient insight into the other characters' motives, Nostromo remains something of a mystery to him, as the last line of the paragraph makes clear. In this sentence we also see that even a society of two is based on hidden deceit and motives of control.

Compare this phrase also with Antonia's, "Men must be used as they are." Antonia's message was that to accomplish any action, any mission, any ideal, men must be "used," and by following Antonia's lead Decoud has become a user of men. Note that Nostromo, the prototypical used man, is again referred to as a possession, just as to Captain Mitchell he was "that fellow of mine."