Excellent points, DumpsterFire. There is a huge difference between a few selfish individuals who want to make every male look like Arnold Swartznegger and every female like Marlene Dietrich and scientists and medical researchers who want, via genetic manipulation, to rid the world of genetically transmitted birth defects or genetically transmitted diseases so that all may benefit.
The Nazi's were like todays wealthy who seem to be saying "So what if we have all the money? How can that hurt anything?" Oblivious to anything but their own wants and desires and their need for power. Motive is everything. Power can ruin everything.
And of course, you are even more right about not knowing what the outcome of any given genetic change will be. Even evolution doesn't know. At one time, sickle cell anemia was a huge benefit to the natives of tropical areas in Africa because it protected them from malaria. But then the world changed, life spans became longer, the propensity of the genetically causes mutation to kill the person by the time they are in their mid-30's became relevant, because people started living longer. When we play with mother nature, even if we are using it to end the aforementioned birth defects, etc., we are still taking a chance on causing even more serious repercussions. That's why we use the scientific method to eke these things out. Because it has the best chance of noticing any problems via well established processes.
The religious call upon their god or their religion on a regular basis, but then they say things like "Praise the lord and pass the ammo". If there were a god, they could say "Praise the lord and let's sit back and watch him pass the ammo!" But they know better. Words are for their god and their god inquiries. Human action is for what we do in real life. Sadly, egos and bullies do the latter wrong. As with the Nazi example. And the religious who do nothing but trust their god are equally inept, even when nicer.
People with good motives (ridding the world of MD and other diseases) are properly motivated and using scientific methods to find a solution. Not prayer. Not natural theology. There is a reason for that.