The Red Shirts of Star Trek are loved for falling easy prey to strange horrors. The Stormtroopers of Star Wars are revered for being the objects of heroic violence. The scream queens of horror are the sexiness thereof. We admire our heroes and respect our villains but we relish our victims.
Sexuality and violence are intrinsically linked. Easy violence is casual sex. An anonymous victim is a one-night-stand.
Killing is thrilling and fiction is rarely intense without it. The silencing the sentry is an unusual theme in that it is meant to thrill but calmly. It is a predator-prey scenario in which one character easily and impersonally subdues another. The distinct appeal is the contrast that what is unceremonious for the assailant is tragic for the victim. The ease of the kill is an inglorious fate indeed.
Monsters are masculine even if female. Victims are feminine even if male. The hapless Red Shirt, for example, is the "vagina" and what kills him is the "penis" in what is thematically the heterosexual dynamic. Such would not be the case if the Red Shirt enjoyed a fighting chance. He is "helpless" because the feminine is anatomically passive. The "penis" does not require consent to penetrate. The prey must be vulnerable by its nature. The victims must be lambs before gathered for the slaughter.
A foot soldier of villainy is an expendable agent of someone else's evil. A bystander is a faceless face in a crowd. We are amused by the deaths of the nameless because their lives are inconsequential. They cringe and scream fearing a doom that shall make no difference. Their bodies are heaped into piles to be lost in the crowds from whence they came. In a strange way they deserve their fate... because they are nonentities.