Simply because the question posed is designed to attack Christianity does not mean an answer to it pre supposes Christianity. My point was not that Christianity is correct, indeed the first thing I do is dismiss Christianity's view on morality as being philosophically unsound. I merely purport that the existence of a 3O god is not inconsistent with the existence of what is perceived as evil.
What do you think "omnibenevolent" means, if not that God should maximize the occurrence of what we perceive as good? If good and evil are ill-defined, then so is omnibenevolence, so God isn't omnibenevolent. If it is well-defined, then evidently omnibenevolence is meant to match up with that definition.
Imho, it is impossible to use logic and philosophy to prove whether god exists (yes I do think Descartes made some illegitimate logical bounds to reach his conclusions), or doesn't exist.
Of course, but that's because existence suffers from similar flaws to those you attribute to good and evil - that is, there is no universal definition of existence. For instance, if it is impossible to observe something, does it exist? What about things that are semantically different but not functionally different, such as various equivalent space-time models? In the end, the existence of God is a fruitless question - the right question is: is it useful to believe he does?
Further, as a second point, even if you assume the universal definition of good does exist by virtue of God being the only being capable of comprehending it, then you cannot assume that evil things actually happen. Since only god knows what things are evil, we can only guess, and if we think something is evil and happening in the world, then we have guessed wrong.
Terms such as "good" exist for one unique purpose: communication. There is no other purpose. The very notion of a universal definition is an exercise in silliness - the quality of any instance of language is measured by how faithfully it can transfer an idea from a speaker to another, not accordingly to some "universal definition". As you said yourself, such a definition would be unusable.
Basically, any discussion about what evil means that doesn't involve two or more speakers trying to transfer ideas between themselves is complete rubbish. Language is an abstraction of brain representations, God wouldn't have anything to do with it even if he wanted to.
The other problem is it is a circular argument. What you want to say is that if there is a universal definition of evil, then god cannot exist.
I think you misconstrue the argument. No useful argument ever depends on definitions, at least not when reasonable people are involved. The argument of evil should thus be understood more pragmatically: the point is to identify an inconsistency in one's belief system under their own definitions. The morality of most humans overlap considerably. I simplify a lot, but say P believes God is good, that murder is bad, and that God murdered people, then P's belief system is inconsistent. It doesn't matter at all what good and evil really mean, heck, it doesn't even matter what you believe they mean. All that matters is what P believes and whether or not P's beliefs are consistent - and if they are not, you have an angle to attack from. The whole art of arguing against someone or a group of people is to pit their beliefs against each other until they realize they can't have their cake and eat it too. The argument of evil hinges on the presumption that God's omnibenevolence and the presence of evil, as commonly understood, are at odds.
Since a definition's only purpose is communication, it can only be an obstacle to idea transfer. Sometimes I admit it can be a heck of a puzzle (or plain impossible) to get abstract ideas through to someone. The main reason why this is the case, I conjecture, is poor internal categorization of concepts: when one's idea of God is inextricably linked to one's idea of morality, and your argument depends on triggering these ideas independently, you end up with no linguistic recourse. I don't think there's a magical way to change another person's understanding of language so that there's a way for you to tell them what you mean, it seems to vary from a person to another.
Let me summarize: an argument is never about being right. An argument is about making others see things your way. So when you say the argument of evil presupposes that there exists a universal definition of evil, you are wrong. According to my understanding of good and evil, God is not an omnibenevolent being. Since I believe that my intuitive understanding of evil is fairly standard, clearly, appealing to it seems like a good angle from which to convince others to think my way. See, I don't care about objective or universal definitions of good or evil. I think I got neat ideas, and I'm using language to push them - I'm not trying to tell others the truth (whatever that means!), I'm trying to make others think what I think, using various weapons I enjoy using, and know to be effective, such as thought experiments, analogies, logic and so forth.
But when you define good- and by extension evil- to require God's existence, then showing an inconsistency only shows that your definition of good is flawed and nothing about god.
A definition is only flawed when it fails to convey the intended meaning from a person to another.