I would say the most important aspect of morality is empathy. Some have it some don't. Usually crime is identified as something the majority empathize with.
My theory is that empathy is the basis of morality, and that stems from survival instincts.
The majority of a population thinks if someone can do harm to others they can do it to me or my offspring.
The majority have made a group effort to survive.
This doesn't just apply to murder.
If the population required no education, women would be getting married very young, as they did in the past. But today, women require the same amount of education as men in order to survive. Parents and grand parents know this. So the group makes what was perfectly moral
into something that is extremely immoral, and by that I mean Marrying a 14 year old girl.
If a man 300 years ago married a 14 year old girl, it was because the parents of that girl knew that older man had the money and land to support that girl threw life, and it was a burden off a poorer family. It wasn't like the girl couldn't see her family ever again, and in her new position she could loan money and help to her parents. The church in those days was very much in control and marriage was sacred and divorce was extremely uncommon, men would have to convince a third party that their wife had been unfaithful and even in those circumstances the church would not nullified a marriage. So men slept around while married women stayed at home.
Now lets look at the same arrangement in today's society.
The church doesn't have a monopoly on marriage, the state does, and the state today doesn't have a very religious view on marriage. It's just a legal contract that can be broken, in many cases resolved by splitting revenue and property, and if kids are involved require a man keep up financial obligation's towards his offspring, but child support and alamony have to be exorbitant to financially support a woman, and those payments cease after a set amount of time. Whether alamony is for 5 years, or child support ending when a child reaches the age of 18.
a woman will still require an education to earn income herself. The benefits of marriage aren't as life bonding as they were 300 years ago. If a woman marries a man at the age of 14 and starts having kids, then her education is in jeopardy, her ability to function in society is in jeopardy, if the marriage ends. Which in today's world is a likely scenario, she could find herself in a situation where she has no way to support herself. So society changes, what was once mutually beneficial has become poison, not only to young women but the society as a whole. Therefore it has become immoral.