10 Star Trek Villains Who Killed The Most

In 2002 (in the Trek universe) Earth launched Nomad, a highly sophisticated artificially-intelligent space probe with the mission to find new life. Unfortunately, in 2005, Nomad was hit by a meteoroid and lost contact with Earth. While aimlessly wandering through space, the probe eventually came into contact with an alien probe called Tan Ru. Somehow, the two units merged programming, which turned Nomad into a sentient being with a new directive: the destruction of anything it considered imperfect.

With this new programming, Nomad went across the galaxy destroying any life it encountered. In the Original Series episode The Changeling, the Enterprise rediscovered Nomad after it killed the four billion inhabitants of the Malurian system, but it's likely that this is only a part of the probe's total death count.

Kirk was able to convince the probe to self-destruct before it could destroy the Enterprise. The probe sought to kill anything that was imperfect, but Kirk pointed out that since it mistook him for it's creator, Roykirk, Nomad itself must've been imperfect.