Some theists believe that we shouldn't tamper with God's perfect creation, because everything happens for a reason (including miscarriages, birth defects, and genetic diseases.) Atheists might believe that natural selection has got us this far, so it must be a pretty good process. A scientist shouldn't change anything, because they will be adding unnecessary risk to the natural process of evolution.
But you have to think about the "baseline". Natural selection is actually a brutal and horrific process where genes are just mutated at random. 50% of all pregnancies end in a miscarriage. Birth defects occur in 3% of all live births, and they account for 21% of all infant deaths. Nature is already a mad scientist that runs random experiments on babies.
I read that humans have about 30 genetic mutations per generation [1]. So imagine if a scientist picks 1 gene at random, and they use CRISPR to randomly change that gene. Now the embryo has around ~31 random mutations. Isn't that basically the same result as natural selection? You probably wouldn't even be able to tell that the scientist had changed anything.
Is that an accurate way of looking at it, or am I missing something?