We can ask not just whether organisms found in nature are good for us, but whether certain human behaviors that are considered natural are also good. Displays of aggressive behavior seem to be quite natural among many animals. Humans can also behave aggressively, and this behavior with its accompanying emotions is surely an evolutionary inheritance, what we sometimes call our animal nature. We can confidently say that aggressive behavior is natural for humans, unlike, say, eating with a fork or chopsticks, which are cultural options. But if acting aggressively is natural, we can ask if it's a morally good thing. There may be cases when aggression is good or at least morally justifiable, as when a mother acts with hostility toward someone who poses a threat to her children. But all of us have been in a bad mood or have just had a bad day, and we've all lashed out at someone for no good reason. In such cases acting aggressively would not be morally good behavior.
There are a host of natural or animal impulses and behaviors, such as anger, jealousy, fear, and lust, that can affect how we treat others, and it's obvious that acting on these feeling is not always a good thing morally.
If it could be shown--however unlikely--that rape and murder were natural human behaviors that are part of our evolutionary inheritance, we can still ask whether these behaviors are morally good.
It should be clear--to most of us, at least--that these sorts of behaviors are morally wrong. Therefore, it would be incumbent on us to combat them by teaching children from an early age to resist the urge to act on such impulses.
Ultimately, we must reject the assumption that just because something is natural, or even a result of biological evolution, that it must be good and worthy of imitation. Thanks to culture, humans are rather flexible and adaptive, and we can learn to resist many unethical behavioral tendencies that may have an evolutionary basis.