The issue of morality is used by those selling religion because most people only know what events transpired during their lives, and within their limited locale. The fact is, what we call "morality" is a series of highly subjective rules that benefit society the most.
Killing is a perfect example, murder is kling that is against the laws of society, and we kill a lot as a society. In self defense, or defense of a weaker being, killing is generally considered okay, and laws reflect this.
In the past, we would kill for grudges, anger, resources, and religion. Today we kill far less for resources, and this reason for killing is now considered immoral, thus it has always been subjective.
The reasons we now consider many acts to be immoral is because it harms society, our species, and slows progress. So we developed alternatives to these acts, for resources we use commerce and trade instead.
Let us consider the harm kling causes. Suppose you contracted cancer, and in some random shooting the person on the verge of curing that cancer was killed.
That is a direct harm caused to one's self by the killing which illustrates the selfish point here, but we can stretch that further. Perhaps the one killed develops a machine which then leads to the cure for your cancer.
Now we are stretching the thought track beyond what dealers of the drug called religion can even consider. The machine to cure analogy is also the most common event in history.
More often than not, a small discovery cascades into larger ones, using critical thought produces those small steps. The term used to describe this is the butterfly effect.
Now consider the instinctual drive to protect your progeny, to promote your genetic chemistry. This requires you defend your progeny, and thus if they got the cancer later, you could have inevitably prevented a cure by not defending the one who was killed.
Thus, we form laws, society supports itself by discouraging any activity which weakens it using these laws. It is preemptively satisfying your instinctual drive to survive and propagate.
If you look closely at everything we call morally wrong for which there is a rationale, you will see it all impacts your chances of survival.