Final answer:
The abusive sergeant from J Company who lectured the men at the seawall is a character involved in executing a condemned man, as part of his duty during war.
Step-by-step explanation:
The abusive sergeant from J Company mentioned in the text appears to be a character involved in the execution sequence, exemplified by the scaffold setup for a condemned man described in the passages. The sergeant's role is a grim one, coordinating the removal of the plank upon which the condemned man is standing, ultimately following the captain's orders to execute him.
No specific name is given for this sergeant in the provided excerpts; he is simply a functionary executing (quite literally) his duties. The depiction introduces a moral question regarding obedience to orders and the impersonal nature of war, where individuals can commit acts of great inhumanity under the guise of duty.