There are many other things that evolve too: ideas, memes, viruses, and many more if you relax "evolution" to simply mean "progress", like my own knowledge and the capabilities of the human race. It's myopic to focus on genes alone.
> eugenics
Eugenics takes for granted the idea that some "genetic purity" is good for its own sake. Why? Why is a world with fewer, but "genetically superior" people, (and more genetically similar), better than a world with more people and more genetic variety? That's not even necessarily good even if you myopically focus on the actual gene pool, much less if you take all the other aspects of humanity into account.
> So, how exactly do we know which is good, which is bad? Is there even an objective good or bad without invoking the idea that some higher beings say so?
Yes, there is: we all collectively decide properties of the world we'd like to live in, and we try to guess what kind of world future generations would also like to live in, and then we call those actions that ultimately improve those properties "good" and those that lessen those properties "bad", and we can even partially rank actions based on the integral of their effect on these properties from now until the end of the universe (allowing a Net Present Value calculation to avoid infinities, to acknowledge our imprecision of future outcomes and preferences, and even to simply prefer utility now over the same utility later). We try to maximize happiness (in its various forms, some more important than others) and minimize suffering (ditto), taking diminishing returns into account so we don't just make one dude trillions-happy at the expense of everyone else. This has infinite degrees of freedom and requires perfect knowledge of the future, so we know it's not actually possible to achieve, but it gives us a direction to orient our moral compasses and strive to get as close to this perfection as we can.