10 MORE Star Trek Villains Who Killed The Most
4. General Nedar
Though the audience never actually heard this name on-screen, the notes for Star Trek: Picard's first season identify the half-Vulcan/Romulan Commodore Oh as Nedar, who held the rank of General within the Romulan fleet. As a member of the Zhat Vash, her hands were drenched in blood.
It was she who was responsible for reprogramming the synthetics on Mars. Thus, this one individual is responsible for almost twenty thousand deaths, and a planet that remained on fire into the early 25th Century at least.
Nedar's motives may (and that's a big may there) have come from a place of preservation, but that doesn't change the fact that her actions directly led to one of the worst massacres in Federation history. There has been nothing heard from this character since her disappearance after the arrival of Starfleet, though it is safe to say that she is still out there. The immediate threat may have passed, but this soldier infiltrated Starfleet with relative ease. There is no telling what she could do, if she really put her mind to it.