10 Star Trek Villains Who Killed The Most

Including this character on this list may be controversial, as it's debatable whether he killed billions, or no one at all.

In the 2170s, the Krenim Imperium was losing a war against a species called the Rilnar. In a desperate effort to win the war, a brilliant temporal scientist named Annorax developed a time weapon that could completely remove anything (from a speck of dust to a planet) completely from history. After the weapon was fired, history would change as if the thing it fired on had never existed. He fired the weapon on the Rilnar homeworld and removed every trace of their species from time.

The second time the weapon was used, Annorax accidentally removed his wife from existence, which led him to lead a 200 year long crusade where the weapon was fired at target after target, as Annorax desperately tried to restore his wife. Trillions of lives were completely undone.

These events played out in the Voyager two-part episode Year Of Hell, however, at the end of the episode, the weapon itself was removed from history, and Annorax got to instead live a happy life with his wife (though, presumably still under the threat of imminent destruction from the Rilnar). Because of this, Annorax in the so-called 'prime timeline' didn't kill anyone (as far as we can tell), but there exists a hypothetical, short-lived, pocket timeline where he killed trillions (if you count being removed from history as being killed).

That's a lot of caveats for our number one spot on this list, but time travel tends to complicate things.