What is immoral about eugenics?

It is a "given" in discussions of genetic engineering that no sensible person can be in favour of eugenics. The main reason for this presumption is that so much horror, misery, and mayhem have been carried out in the name of eugenics in the 20th century that no person with any moral sense could think otherwise. In fact, the abysmal history of murder and sterilisation undertaken in the name of race hygiene and the "improvement" of the human species again and again in this century is so overpowering that the risk of reoccurrence, sliding down what has proved time and time again to be an extremely slick, slippery slope, does seem enough to bring all ethical argument in favour of eugenics to an end.