well, the idea of eugenics was basically this.. Selecting only the humans with the best traits to pair up , so that after a few generations everyone gets smart, sexy, strong... But the downside of this idea is that fat bastards like me would be put down (killed) since i guess that I don't fit their idea of a perfect human

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There were two problems that I can think of.
1: Fat/Ugly are subjective. If you took the top 10% of slim people in the world and eradicated the rest then within a generation or two the people at the top of the new "fatty" scale would be considered fat.
2: If you reduce the size of the gene pool you run the risk of having no human with a suitable DNA mutation to survive a new environmental change (such as a virus), and increase the number of occurrences of genetic diseases.