“It is forbidden to kill; therefore all murderers are punished unless they kill in large numbers and to the sound of trumpets.” - Voltaire
The ethics required to function within modern society are artificial. By artificial, I mean they are constructed by people in order to serve specific functions within groups/societies. This artificiality makes ethics fluid and relative. It is easy to demonstrate the relativity of ethics. Many of us believe that killing is wrong but accept the killing of members of out-groups for the good of our in-group. Additionally, in most societies, it is deemed acceptable to kill another human being if you are protecting your life or the life of your family/friends. Therefore, the ethicality of our behavior has to be interpreted within a specific context, making ethics relative.
“Though shalt not kill” is not sufficient in guiding ethical decision making because it doesn't address the many situations where killing would be the ethical thing to do. For instance, it would be unethical not to kill an individual who was planning to set off a nuclear weapon, when killing them is the only option to stop them from carrying out their plan. It would be unethical not to kill an individual who is suffering tremendously, for whom there is no cure for the suffering, who has stated that they want to die but are unable to carry out their own death.
The above two examples are of course my personal opinions for what is ethical and what isn't. There are individuals that are absolutists and believe that killing is wrong under any and all circumstances. But this absolutist stance on killing is not evolutionarily advantageous/sustainable. Imagine that group A believes that killing is wrong under all circumstances and refuses to kill other human beings. Imagine group B accepts that killing anyone outside of group B is acceptable and virtuous. Group B would quickly murder group A out of existence. The only way for group A to survive would be to modify their ethics to at least allow for killing of others for selfdefense.
If relativity plays a role in determining the ethics of killing, the flood gates are wide open for other seemingly lesser offenses such as lying, stealing, cheating, torture, etc. Therefore, it is important to consider context and to periodically reevaluate the ethical frameworks order to determine whether they are still relevant and serve a progressive function.
As gene-modifying technologies, such as CRISPR/Cas9, and general artificial intelligence approach maturity, it is more important than ever to consider the ethical implications of these technologies for the future and to modify our current ethics in order to survive the coming transition.