There is nothing particularly wrong with eugenics in itself; it is how some people choose to carry it out that becomes questionable. When eugenics is used to propagate racist or dumb ideas.
This makes several assumptions not supported by science. Eugenics presupposes that genetics alone is a significantly large factor influencing traits like intelligence and "predisposition to criminality" that you can just selectively breed humans like farm animals and thus improve the whole population over time.
If you dig deep enough into the history and science, that's not necessarily true, and even to the extent it is is fucking complicated. It ignores or discounts environmental factors, cultural norms, and probably more I'm not thinking of atm.
Picking general intelligence, however that's defined, eugenics says you need to get two highly intelligent people to produce offspring and thier children will automatically be smart. In reality they might have an edge, but that edge can be lost if the kids aren't provided with a stable home environment, healthcare so they can thrive, high-quality education, etc. It doesn't mean shit if your kid is the next Einstein according to an IQ test (which are fraught with problems by themselves) if they can't read.
The topic is higly debated by people much more educated than I who spend thier whole lives studying the topic. I'd encourage you or anyone interested to do some reading on the subject from actual experts (which I, for the record, am not) with backgrounds in genetics and especially anthropology. "The Myth Of Race" by Robert Sussman was an incredibly eye-opening book for me personally that obviously focuses most on race but eugenics gets covered too.
Fair enough. I took your original comment a certain way, probably cause the way it sounded is the way a significant potion of the population thinks. Physical variation is a much more understood (and less dangerous) thing too.