If by "a single moment" you mean "in about a hour or two" then I'd guess it was the Boxing Day Tsunami on December 26, 2004. Most people were drowned or battered to death within moments of the water hitting them, but the water hit different places at different times depending on one's distance from the epicenter. It killed 230,000 to 280,000 people.
On the other hand, the Bhola Cyclone in Bangladesh (then known as East Pakistan)&India killed about 500,000 people on November 20, 1970. That all occurred in one day but over the course of several hours so not exactly instantaneous.
The deadlier of the two atomic bombs was Hiroshima's, which killed about 66,000 people pretty quickly, but not instantly--a firestorm was created that took a short time to kill many of the people, though how many in that area hadn't already died of the bombing, no one can say. You'll see figures for deaths at Hiroshima up to 166,000, but most didn't die right away, but lingered on for days or weeks or months.
Well lets see. Assumeing you mean litterally instant vanishing of life, not flooding, drowning, burning, etc. We can list a few things.
Starting from small to big lets look at a few examples.
During WW2, the British navy hunted the German battleship Bismark. They sent the HMS Hood one of the "jewels" of the fleet to sink her. During their first and only battle a shell from the accurate Bismark penetrated the deck and exploded the magazine. In a second all 1408 members of the crew died save for three.
During WW1, at Messines the allies planted a network of mines under the german trench. When it exploded 10,000 germans died.
At Hiroshima it is hard to get an exact number of deaths but it is fair to say that it was more then 10,000. From the blast and the firestorm it is estimated at 70,000 to 80,000 died. The question is how many died instantly versus to fires/radiation/suffocation and debris.
Now that is man made deaths.
Lets look at Mother nature.
Mount Vesuvius when it exploded in 79AD is believed to have killed 16,000 people instantly. It was at first thought it was suffocation but now it is thought that it was a massive 250 celsius heat wave that killed everyone instantly.