I noticed some individuals say that there is no such thing as good and evil. Why exactly do they say that?

I'm one of those people. What I mean when I say it is that good and evil isn't black and white there are a lot of grey areas.

Don't remember what it's from but there a quote that states "every villain is a hero in his own mind." That's also what I mean when I say it. Some 'bad people' genuinely think they are in the right. War is a good example; each country thinks their side is the 'good' side.

Good and evil are just words. Humans have taken to using these words far too lightly, in a systematic, yet chaotic world of nature. It only applies to us, because we created our language and words to match our feelings and descriptions, which can only be applied to us, as the rest of the world, that isn't man-created, has only natural law applying to it, rather than the basis of our words, which only mean something to us. I'm not saying that we are necessarily wrong in anything we describe, but that we aren't always right, either.