Depends on how you define evil though. If one assumes that evil can only be defined in contrast of good (and vice versa) then evil can be viewed two ways: as everything that does not fall into the category of good or as everything that is the opposite of good (thus implying a middle zone). Such a construction of evil presupposes a defined good though.

Even with that model, there still remains the problem of defining good (therefore defining evil). Good can be measured by various things and values. In various systems and social context, different values are given a different priority. Thus, a definition of evil is dependent on that context. One could say, to judge the evilness of the villain, one first look at the context which it is in. Is the villain truly evil within the society he is functioning in? I mean, if the society emphasizes law, order and logic over all else, then someone who is as chaotic and random as Kefka would be truly evil. Yet in a society where say balance is regarded highly, Kefka may be only seen as a necessary balance to the excess good in the world therefore not so evil.

If we are to measure then their evilness by the context of their society, it then makes it difficult to compare villains. Sure, each villain could be evil (to various degrees) in their own society and in other, similar, societies but it would make it difficult for cross-social comparisons.

Therefore, one can conclude that the only way to effectively to compare two villains from their respective social settings is then to create a setting of agreed upon values and use that as a comparison for their deeds.

Then again, what I am saying is merely only functional given the fact if you define evil in a contrast, assuming a intradependency with good.