OK, let's look at how that works in the case of sickle cell. In a population where many individuals carry a single copy of the sickle cell gene, assuming they live in an area where malaria is common, more individuals are able to survive and reproduce.
Those individuals with two copies of the gene may not live to reproduce, but the gene is nevertheless selected, because it benefits the population.
Other, perhaps better, examples have been presented.
They are dead before they reproduce The gene is gone unless it resides in other individuals who aren't killed by the defect. Individuals are selected not populations. You chop down trees. You cannot chop down a forest without chopping down trees.