People act like moral questions are difficult. There are some vexing moral questions, but not many.
Kant explained it very simply: “Handle nur nach derjenigen Maxime, durch die du zugleich wollen kannst, dass sie ein allgemeines Gesetz werde.”
We say an act is evil when the actor who commits it would not want to the world to be such that he could be the subject of that act. A murderer does not want to be murdered; a thief does not wish his own property to be stolen.
A cat kills a mouse she has no intention of eating and moral people are repulsed, but we do not call the cat or her behavior “evil”, because the cat cannot conceptualize what it would mean to be the mouse in that situation. You and I can, and therefore we are subject to moral laws. If we violate them, we are, in fact, evil.
There are some people who do destructive, unlawful things to others that they would cheerfully endure themselves — but those people are insane, and not evil. They are detained, hospitalized if possible, but not imprisoned or executed.
Imagine the only possible choice in life were to drive on the left side of the road or the right. Which one is chosen is utterly arbitrary, but a world in which each driver made the capricious choice to drive on whichever side struck his fancy would be a world where the roads would be dangerous to the point of impassible. There has to be some rule that everyone follows.
Thus an arbitrary choice — made by majority vote, by some bureaucrat, or just by custom — becomes a moral imperative.
Notice that the Categorical Imperative arises without any reference to specifics. If everyone in the world desires to be protected by a rule, then everyone in the world is subject to that rule, whatever the rule is.
The specifics depend on the population being governed. It would be interesting to speculate what would happen if there were two intelligent species on a planet, and the members of one species had no objection to dying and therefore regarded killing or being killed as being of no more significance than touching or being touched on the shoulder, while the other species felt as humans do on the subject. How could rules be established to govern everyone?
In the absence of such species though, knowing how to live a moral life is pretty simple, even in the absence of a supernatural lawgiver, although actually doing so is not necessarily easy.