When we think of sin, we think of things like murder, envy, strife, wickedness, rape, cheating, hatred and more. All these things are considered sin but they are just symptoms. At its root, sin is simply turning away from what gives us the most joy: God himself. Listen to the words of Jeremiah the prophet.
...for my people have committed two evils: they have forsaken me, the fountain of living waters, and hewed out cisterns for themselves, broken cisterns that can hold no water. (Jeremiah 2:13)
God says that his people have committed two great evils. First, they have forsaken God. He is the fountain of living waters, he is our source of life. Yet, his people have turned away from their source of life and instead, committed evil number two. This second evil is the act of digging out our own cisterns. Cisterns are pits carved out of limstone rock and plastered to prevent seepage. Runoff water flows into such pits and collect there. Often, there is silt, mosquito larvae and dead animals in cistern water. But what's worse, is that God's people have broken cisterns! There isn't even water in the cisterns we have hewn for ourselves.
In other words, we have turned away and dug into the ground with our hands, thinking we can find a better alternative to the Living Waters that is God himself. Instead, we end up eating dirt. We turn away from what is best for us and create for ourselves lives of misery. This brings to mind what Adam and Eve did. They had experienced God, their Creator and sustainer of their lives. They had experienced his goodness and provision. With him, there was happiness. But they turned away, chose their own path and looked for life elsewhere. Their life ended in misery. (Genesis 3)
Turning away from God, our source of life, is the first great evil. And the second is similar: carving out our own miserable existence. We dishonour God when we turn from him. That is the essence of evil: turning away from God.