Wow, very interesting discussion this has led to.
It seems like what this all comes down to, and why there can be no "absolute evil," is because
good can survive without the existance of evil, but evil cannot survive without the existance of good.
Since evil, is a "rebelling against good," then there must be good for evil to exist. Society can deem "good" to be whatever it wants, and evil would be the "striving against good" or the "rebelling against it." Which means there can be no "absolute evil," since that would mean there is no good, and evil cannot exist without good.