Theory: That Eugenics war, or Khan and his friends, is indirectly caused by Dr. McCoy

https://www.quora.com/Do-you-have-any-crazy-fan-theory-about-Star-Trek/answer/Sooraj-Vijaya

Remember this lady?

https://qph.fs.quoracdn.net/main-qimg-eec87586b527f6004eacbf09ebf928e5

She is from Star Trek IV, Voyage Home. She suffers from kidney failure and she is a dialysis patients in Mercy Hospital.

Bones, being a doctor tries to save her by giving her 23rd century medicine that made her grew new kidney. This baffles her doctor and of course when will then be a subject of multiple medical test including blood tests and more.

The scientist of 1986 then found a strange substance in her bloodstream that controls cancerous cell and manipulates it's genetics to enable creation of new organs. Reverse engineering the the substance was done in imperfectly or incorrectly applied, but good enough to make eugenics born. However the tiny flaw on the reverse engineered substance was affecting the brain of the eugenics and that made the eugenics evil.

It's simply impossible to create eugenis back using 1986 / early 1990s technology. But when the the scientist can reverse engineer medical technology from 23rd century technology, it became possible.

So yea, saving one person resulting in killing thousands.

Edit 1: thank you for all the comments, as someone mentioned that eugenics program was started many years before 1986, I guess the 1986 Earth medical technology was accelerated to such a degree by 23rd century medicine that made the eugenics viable.

Resulting in enhanced power, faster reflexes, faster growth and more.