Sin then is any willful disobedience to the known will of God. I've heard it taught that if we thought about a deed it was the same as doing it. If you believe that the next time you walk by a big dog and think about kicking him, just go ahead and do it. When a thought comes to our mind we must consider it then decide what we will do. What we do should be based on what God says and not on our desires.
Sin pays wages.
The wages of sin is death. That means if we sin we are working and the payday will be death. Sin always carries us further than we intended to go, costs us more than we wanted to pay and causes us to stay longer than we intended to stay. I know because I have been there. To those that sin, sin becomes their master.