At the most literal level, yes, moral terms like good, evil, right, and wrong can mean whatever we want. Why? Because moral terms aren’t special. Any word can mean whatever we want. This should go without saying, but languages are invented and maintained by the population of speakers. If enough speakers evolve English so that evil means good, that is how it shall be moving forward. A single speaker redefining words will find himself unable to effectively communicate with everyone who hasn't adopted his fringe definition. That's why the meanings of good and evil tend to remain roughly the same within cultures, secular or religious. It’s hard to get everyone on board for an arbitrary remapping of terms.
The secular definition of moral good is essentially to behave in a way that is beneficial to others. Regardless of the letters written or sounds made to communicate this concept, it would not change it’s value. In other words, if “good” is renamed “evil” tomorrow, that would just mean that we would start to value “evil” over “good.” Now the question is: do we value the concept we call good because a divine third party made us? There’s no way to know, but we do know that there are reasons for social beings to value the concept we call good independent of supernatural mandates.
- Being good to others earns more opportunity for earn friends and make families. (Here I could argue that the odds of finding mates and living longer are increased with this behavior, making the instinct to be good a selected trait, but I think this argument is only additive and not required for my points. Since people who do not accept evolution will likely read this, I will not argue this further.)
- Being good allows for the creation of culture and societies that provide benefits ranging from the division of labor to shared resources.
- Being good, rather than evil, keeps animosity from others low and makes for a more safe and stress free life.
There are many more reasons to be good rather than evil, but evildoers still exist. Apologists argue that people who do wrong do so because they separate themselves from God or ignore that divine moral compass within them. Some even say that it’s an acceptance of moral relativism that swings open the door to sin. While it is always wrong for the slave to strike an owner, it may be right (in the owner’s mind) for the owner to strike a slave. The rapist isn’t wrong to rape because (in the rapist’s mind) what’s right and wrong are up to whatever we want and, the rapist wants to rape. While there are individual defectors who periodically discard their value of good to serve base desires, they are rarer exceptions. More common are those who maintain their value of good with a more narrow view of equality. The slave owner treats other whites as peers with the same understanding of good as you or I, but define blacks as a class in which the definition does not apply. The same could go for the rapist and how he sees women. It may not be the definition of good that’s a moving target, it could be the definition of human.
There is more to the secular meanings of good and evil after we take into account context, motives, consent, ect. What’s most important to the point is that every culture has a name for this concept and places a high value on it. Apologists who admit this attribute it to the aforementioned God-given moral compass within us. I’ll stick with the alternative that all humans share something else, a desire to not be alone. Don’t underestimate how lonely, and short, one’s life would be if they placed no value on being good.