If I say that red is between the wavelengths of 622-780 nanometers and you say it is between 621.5-780.5 nm, who is right? Can we concretely define 'red'?
Yes, my measurement for red is observer dependent (or relative) to my sense impressions, but the important point is that others understand what I mean by red when I say 'that color is red'. Once you understand what I am calling red, you can see if I am consistent in that labelling. For example, if you showed me a color reflecting light around 618 nm, and I called it red, then you can say that I am being inconsistent with my definition. I might have to revise my definition or deny that I really meant 'red' when referring to 618 nm (maybe I meant the color is reddish, etc).
***We probably can define "evil" relative to a certain obsever given a certain criteria for it (say, for example, any opinion beyond a certain "percentage error" from yours) but it's still completely relative to the observer. So, I think trying to concretely define it is futile. Especially since ya can't really quantify emotions or feeling.***
Taking the above illustration, which is really relative here? The definition of red is relative to the way I communicate my sense impressions, but notice that this is the case anytime anyone communicates. We are stuck in a world in which we must somehow take the ideas in our heads and compare and contrast them with the ideas in other people's heads. This is try even for emotions and feelings. Often in relationships we are asked to communicate our emotions and feelings. What we are doing in such situations is defining the way we feel with words that we think mean something similar to how we feel, and then we try to use those words in some type of combination whereby our wife (girlfriend, partner, etc) will understand our feelings. This is the purpose of language.
I think what happens is that we often communicate so well in normal circumstances that we might fail to recognize that a word is just a symbol for something of shared experiences. When we speak of 'red' we don't feel the need to define it because we just assume that everyone has such similar experiences that we think the word has an automatic objectiveness about it. But, as a scientific experiment, we could easily show how even a discussion of what is red is often construed differently by others. Every word is like this to some extent.
The words evil, good, etc are no different. They have many meanings and if you really want to understand their meaning in specific circumstances, you must revert to the essence of language which is trying to understand the speaker just like you might have to do when trying to express some abstract feeling you might have in a relationship.
My answer is a little longwinded, but I think this subject is an important one. Language is always relative to the intent of the speaker (as well as the listener since half of the communication is interpretation which holds a whole other ball of wax). We just happened to focus on words like good and evil, but in actuality every word in any language ultimately falls in this ballywagon.
Warm regards, Harv