Sin is vice or viciousness - the absence of virtue.
Virtues include love (real selfless universal love), peace, joy, humility, compassion, honesty and so on - there are many other virtues, and they are all characterised by selflessness, unity, and freedom. Virtues are qualities of the soul, and they are innately natural to us.
Sin is the inverse of these sorts of qualities, such as hate, violence, greed, arrogance, cold-heartedness, deceit - there are many more sins also, and they are characterised by selfishness, manipulation, and restriction. Sin is alien to the soul (albeit natural to our animal bodies), and an accumulation of sin blocks out the natural virtues of the soul.
Virtue elevates us and makes us more happy and contented, loving and caring, peaceful and harmonious. Virtue attracts virtue. Being virtuous attracts more virtue to us.
Sin, on the other hand, debases us. Sin is self-destructive and masochistic, and makes us fearful and miserable, cold and ruthless, hard and vicious. Sin attracts sin. Over time, we become lost and tormented souls.
In these times, the human race is increasingly dominated by sin, especially by those who seek to dominate us. Capitalism promotes and rewards sin. Our political systems promote and reward sin. Our cultural systems increasingly celebrate sin. All of this can only end in self-destruction, misery, and pain - and we can see that happening now all around us. The solution is in our own hands: renounce our sins and embrace our virtues.
Sin is literally self abuse, as every reaction produces a counter reaction from the cosmos. We get what we give. There are always consequences of the same nature and in direct proportion to the viciousness that we perpetrate. This is natural cosmic justice, and it operates immaculately whether we believe in it or not.