For one thing, the idea that there are “bad” genes stems from the outdated idea that there’s a one-to-one correspondence between genes and physical traits—but the reality is that most genes govern hundreds or thousands of traits, and most traits depend on similar numbers of genes. So bad traits usually result from the wrong combination of genes that are not bad in themselves—take sickle-cell anemia, which results from the wrong combination of genes that by themselves offer malaria resistance. (Most cases are far more convoluted than that.) If you remove the genes that cause the “bad” traits, you’re also removing the good traits they cause in other contexts.