So if we are talking about ethics, the explanation for why people do what they do from an evolutionary standpoint doesn't say much about the moral implications of it. There are probably a lot of evolutionary theories on anti-social behavior in humans, say. Maybe some people are inclined to kill people in a surge of passion because that is some latent fight-or-flight instinct that has been beneficial for the survival of the organism, yadda yadda yadda. So what, though? That is not interesting when it comes to considering what is ethical and what is not.
It may seem that I am bringing up extreme cases, when procreating is seemingly benign. There are still considerations that one might have, though; even if it is beneficial for the propagation of the organism's DNA to have a certain amount of children - maybe for example 5 is the best in order to make sure they grow up relatively well - it isn't necessarily ethical. Maybe 3 children is the best for a happy and full childhood, with enough parental attention. Maybe bringing children into the world isn't something that a certain person should do, because of some terrible genetic disease (now we're straying into eugenics, and we probably know too little about DNA to be able to say that so-and-so should procreate and so-and-so should not). Propagating DNA shouldn't even be a goal in itself, ethically speaking. Humanity might have done more harm than good in becoming 7 billion people. And I don't just mean bad for each other, but in causing stress to ecosystems and the global climate which lead to natural disasters and other organisms struggling to adapt and survive.
So what if there is some evolutionary basis for some behavior? There has to be some reason. But that doesn't necessarily mean that we should endorse or encourage that behavior (whatever behavior is being considered).
Why did he kill him? Elementary, my dear Watson; first, we have to go back approximately 20,000 years ...
