Sadly, the devil is in the details of determining whether something is a genetic defect, or an adaptation to an environment just around the corner. Would you get rid of cycle cell anemia even though it gives an advantage towards Malaria? Genetic variability tends to be beneficial in the long run. This can easily be seen with dog breeding, where the hybrid crosses have more vigor than the "true breeds".
In the end, one would be justifying a decision based on a bias.
That's rationalization. There are definitely defects that do not have any trade off. It is bias in itself to think that every genetic trait has a balanced positive and negative.
It's the apple and oranges argument people often use with programming languages. They think every programming language is just a tool in a tool box, good for one thing bad for another. They preclude the possibility that certain programming languages can be good for almost everything or that other ones can be complete crap for just about all applications. There is no logical reasoning that precludes either of the aforementioned languages to exist. It's illogical and it's biased to think everything is fair and balanced.
Logically speaking, executing the people with clearly defective traits is overall beneficial to society. There's no other way to reason around this. You can try but it'd be a biased endeavor as your specifically searching for reasoning to counter mine for the sake of creating the contradiction itself.
In the end, one would be justifying a decision based on a bias.