A grainy image of his face drew comparisons to Hollywood heartthrobs. A jacket similar to the one he’s wearing on wanted posters is reportedly flying off the shelves. And the words written on the bullets he used to kill a man in cold blood on a sidewalk on Wednesday have become, for some people, a rallying cry.

Four days after a gunman assassinated a top health insurance executive in Midtown Manhattan and vanished, the unidentified suspect has, in some quarters, been venerated as something approaching a folk hero.

The authorities have pleaded for help from the public to find the person who killed the UnitedHealthcare executive, Brian Thompson, who was a husband and father of two children. But in a macabre turn, some people seem to be more interested in rooting for the gunman and thwarting the police’s efforts.

The Upper West Side hostel where officials believe the unknown man stayed during his time in the city has reportedly received a deluge of bad reviews online, with some people calling the workers there “narcs.” The business has been cooperating with the police.

And while high-profile crimes have in recent years mobilized internet sleuths hellbent on finding answers, civilian efforts to find Mr. Thompson’s killer have appeared muted. Instead, the executive’s killing has released a tide of online frustration toward the health insurance industry, with some people even voicing their support for the gunman.

It is unclear what motivated the killing or whether it was tied to Mr. Thompson’s work in the industry. The police have yet to identify the shooter, and he remained at large as of Saturday.