This is a great quite from Edwin Black, the foremost historian of eugenics in the US:
Mankind’s quest for perfection has always turned dark. Man has always existed in perpetual chaos. Continuously catapulted from misery to exhilaration and back, humanity has repeatedly struggled to overcome vulnerability and improve upon its sense of strength. The instinct is to ‘play God’s or at least mediate His providence. Too often, this impulse is not just to improve, but to repress, and even destroy those deemed inferior. Eventually, the Judeo-Christian world codified the principle that all human life should be valued. . . Science offers the most potent weapons in man’s determination to resist the call of moral restraint.