In summary it is someone that willingly “violates the principle of non aggression”, which means it is someone who initiates violence against someone or someone’s property.
To understand why this is the metric of evil I suggest reading the book “The ethics of Liberty” from Mises, or “Man, economy and state” from Rothbard, which goes in extremely deep into this question.
But in a quick summary, in order to flourish, societies need to organize themselves to inhibit this kind of behavior. Evil is the label societies put on people who display this behavior, which means that the person is not a desirable member of the society, and needs to be suppressed or cast out.
Societies that fail to suppress this behavior naturally suffer dire consequences. That is why all laws that suppress this behavior are considered “natural laws”.