How do we define good and evil? In a society unshackled by the moorings of a rule set provided by higher power(s), the new rule set would need be based on something. Probably legality. The upshot is that legality can be changed as the rule-makers see fit which allows for some flexibility. The drawback is that legality can be changed as the rule-makers see fit allowing for flexibility. We have a conundrum. Another problem how we determine who determines who the rule-makers are? Is it a simple majority rules type situation? Who protects the weak in a simple majority rules type of situation? Why protect the weak at all? If it is about survival of the fittest then clearly the weak aren't fit so they don't need to be protected.
Question: What if you are deemed to be weak by science and the rule-makers decide that the weak need to be exterminated? If that decision comes down you could be unable to escape this conclusion using science because the analysis has already determined that you are one of the weak. This possibility is why I would not want to live in a world where objectivity rules and logical conclusions are followed without consistent moral consideration made for those who are unable to protect themselves.
Another way to explain this is that if I were to follow a survival of the fittest mentality without some higher moral code to rely on then there is no reason for me to consider anyone who does not help me survive and advance. If you have something I want then I will take it if I can get away with it. If you have a woman I want then I will take her too if I see fit. If you are in my path then I will crush you if I see you as weak. There is no right and wrong. There is only strong and weak and survival. I think the world would rapidly degenerate into piracy. Now, one could argue we are already living in a world where the strong prey on the weak and this is true but I think that faith keeps a kind of governor on the baser instincts of large swaths of the unenlightened, providing order, structure and a safety net for those perceived as weak.
Another issue is that "strong" and "weak" are relative terms and for the strongest there would always be more weak people to take advantage of. I think in this scenario humans would wipe themselves out sooner or later.