Good and evil can't be completely subjective, there must be an objective element to them, otherwise we would have never developed these concepts. Philosophically speaking the good is always prior to evil, and evil is a lack of goodness, falling short of a certain standard. If good and evil were equal but opposing forces, then we could talk about full subjectivity in morality, however they are not equal or even symmatrical forces. Evil can only exist contingent on something good, it is parasitic in nature, while good can exist on it's own and be satistactory as such. This means that even evil people want something good, but they go about it the wrong way.
I think this would naturally lead to the conclusion that the root of evil is ignorance amongst most normal people and malevolence amongst psychopaths.