This sounds like a Twilight Zone episode (https://en.wikipedia.org/wiki/The_Shelter_(The_Twilight_Zone)) but the details are somewhat different from what I recall (specifically, I don't remember the killing)
I know that Rod Serling did remake TZ episodes as short stories, so if you read it as a short story this might explain the discrepancies.
It's also possible that there's an unrelated story with a similar plot to the TZ episode, but if so I've never come across it
EDIT: reviewing the text of the short story version on Amazon, it looks unlikely that this is the same story. The version published in New Stories From the Twilight Zone follows the plot of the television episode, including the neighbors breaking down the door with a battering ram, and does not have a killing or the man guarding the shelter with a gun.
It's possible that the OP's memory has altered the plot over the last 30-40 years.