If you treat sin as some concept defined from other terms, rather than a basic existential reality on a similar level as truth, goodness, unity and whatever else you want to put on the list, you probably aren’t talking about the same things as Christians. 

Of course, you are always welcome to declare that you aren’t interested in talking about the concept and engage in rhetorical practices to convince others to avoid talking about it as well.  And of course, if you are right, than your opponents are clustering around some type of delusion which should be accounted for descriptively.  But, since for Christians sin is fundamental and inescapably part of the experience of being-in-the-world, showing me some definition of it is inadequate does not disprove it. 

The only rational disproof of sin possible, like the other existential concepts, would be a proof of its impossibility.  With a few of them (God comes to mind) such a proof is at least imaginable, though with others I’m rather dubious whether such a proof could really be sustained.  And Sin seems to fall into the first category for a non-believer; and I certainly encourage you to argue against the plausibility of the absolute negation and unworthiness which is one aspect (of many) one can talk about with regards to Sin as a contentful approach to the world.  Perhaps I will turn out to be wrong, and what I think is a reality is mere epiphenomena or impression without content.

However, from where I stand Sin is not only possible, it is actual in my life.  I see it in my constant and virtually inescapable failure to pursue the good that I believe entirely in (even in mundane things) for less meaningful versions of those very same things (superficial exchanges over friendship, tidbits of knowledge over actual integral understanding etc.).  I see it in my constant infliction of injury upon those I care about because of my self-interested pursuits.  And I see it in the fact that I must struggle to even live a life absent horrendous evil; to even avoid being directly implicate in enormous harm to others requires some degree of effort. 

Somewhat more importantly, I see this in others as well, and the testimony of history shows that we are not an age alone in this manner.  Whatever its origin, every person is systematically aimed at failure and self-destruction in their personal life.  Sure, on aggregate we produce lots of cool stuff and knowledge- I entirely believe that humanity is made in the imago dei, and the sheer beauty and majesty of what we create is incredible.  But very few individuals- and sin is in large part a question of our actual existence take this beauty and knowledge and internalize it- they remain marred and deeply broken even as they produce and study things of incredible beauty and power. 

Because I am lazy, there are many aspects of sin to discuss, that I have not mentioned here.  Like Truth, or Goodness or anything else really important and essential to our existence*, talk about these things cannot exhaust their essence because they are the basis for our interactions with objects, and are necessarily present, however indirectly, in all that we do and say.   

*Granted, the inexhaustibly of sin, on the Christian perspective is because Sin is that which blocks the way to our highest End, God.  and just as God is infinite, there are infinitely many ways to deny him.