Tarawa Beachhead - Kerwin Mathews, Julie Adams, Ray Danton 1958

https://www.imdb.com/title/tt0052272/ A marine sergeant witnesses a captain kill one of their men at Guadalcanal to cover his own incompetence, and fears he won't be believed because of rank, so finds the captain has gotten him assigned to his next command plus is cozying up to the family of his victim.