The only argument that can be put in favour of eugenics is an extremely crude utilitarian one, but it is in itself contradictory because accepting eugenics opens up a pandora's box of birth control and social engineering which it is in the collective interest of the humanity to keep securely locked. For instance, if someone with Down syndrome should be rendered impotent, why not force women over 45 to have abortions, seeing as that would greatly reduce the likelihood of Down syndrome in the first place? And in that case, why not force pregnant heroin addicts to have abortions? Etc, etc. We shouldn't debase ourselves by resorting to such methods in any case.