Like “Testament” — the 1983 movie that imagined the fallout, both nuclear and psychological, after an atomic bomb is dropped on American soil — “Leave the World Behind” depicts a plausible doomsday scenario from the perspective of a handful of ordinary characters. Not military experts, not scientists, but two families obliged to shelter under the same roof out in the Hamptons while something scary unfolds a few hours away, off-screen, in New York.

As the stress-prone matriarch of one of these families, Julia Roberts utters a line that earns a big laugh near the start, partly because America’s sweetheart isn’t supposed to say something so dismissive of everyone else on the planet, but also because many audiences can probably relate. Staring out the window of her Park Slope apartment, she pronounces, “I fucking hate people.” The name of Roberts’ character is Amanda Sandford, but it might as well be Karen, such is the rush-to-judgment,...