"He" can refer to "Death" as easily as to Decoud in these lines. Death did come upon Nostromo unawares in the form of Viola, but it also recalls the vulture that "went away" when Nostromo awakened at the fort. That awakening represented the "death" of the old Capataz and the beginning of all his troubles, so in that sense Death "betrayed" him by going away and not finishing the job.