There are so many sickly-sweet goody two-shoes in Starfleet and the Federation — not to forget those no-boot Megazoids — that a cosmic balance with villainy has to be found, if only to keep the universe's blood sugar levels in check. You might like the bubbly, the clawing, and the happy, but put the root beer down: it's insidious! Well, we're still going to need the saccharine salvation of the Federation from the bad guys on this list.
Very few Star Trek foes are out-and-out, one-note monsters. There'd be little point trying to find the light in the likes of Redjac and Colonel Green but most antagonists tend to have at least one saving grace. Trek prefers to find the 'humanity,' even in the darkest depths of deviousness.
Sometimes, however, the deeds of a character, organisation, or even an entire species go so far beyond the pale (of the moonlight?) that they airlock any chance at redemption they may have had, probably along with a few of their victims. This list will focus on a cast of incurables from Star Trek who, no matter any arguments or disclaimers in their favour, can neither be reformed nor forgiven.
And beardless Seán hasn't made a cameo yet, so he can't be number one.
10. The Augments
Very much a product of their time, Khan Noonien Singh and the Augments (as they later came to be called) were Star Trek's exploration of the 20th century's most destructive set of beliefs: eugenics. These particular villains were the result of humanity's experiments with DNA resequencing — the double helix structure of DNA having only been discovered about a decade and a half before Space Seed was first broadcast.
In the late 20th century, the Augments conquered and enslaved the world. The resulting Eugenics Wars between them and the 'regularly' DNA'd had cataclysmic consequences for Earth: an estimated 35 million people were left dead before the dictatorial 'supermen' were deposed. All remaining Augment embryos were put on ice, and Khan and 80-odd of the other overambitious Clark Kents were sentenced to death. They managed to skip town (and the solar system), however, aboard the SS Botany Bay before execution day. The 90s were wild, man!
In 2267, Khan and Co. made an ultimately unsuccessful attempt to commandeer the Enterprise, but Kirk let them off with little more than a slapped wrist, a teeny tiny terraforming project, and one Marla McGivers. Afterwards, neither Kirk nor Starfleet stopped by for a Ceti Alpha high-five, and things went from bad to worse on the planet. Khan, of course, would return to seek vengeance in the film that sports his name.
In that movie, and not before a lot of ear eels and Moby Dick, Khan and the other Augments went the way of the Old Testament. Knowing his death/rebirth as a rock was coming, Khan solidified his status as irredeemable villain by using his last moments to quote Melville's relentless Captain: "From hell's heart, I stab at thee; for hate's sake, I spit my last breath at thee."
Naturally, a couple of members of the Soong family were also involved in all this along the way, but more on that later.