I've always heard that sin is "a willful transgression of God's Law". In effect that's things people do on purpose which God commands not to do. Even formal definitions define it as "an immoral act". Again, the definition points toward things done with aforethought.
Neither of these are true. There are willful transgressions. Those indeed are things people do. Also, are sins of omission which are things people fail to do. Even accidental infractions are sinful, because men are called to do things with sobriety.
With the latter, for instance, if men are rough-housing and one severely hurts the other, it's an accidental sin because those fighting should have considered the potential for harm in scuffling.
Therefore sin is more than willful acts contrary to God's commands, but sin is failure to be righteous. That raises the question, "What is righteousness?" It's doing God's will for right reasons. Valid reasons are for honoring and magnifying God, not to satisfy oneself. In other words righteousness is minimizing "the you" in the eternal scheme of things Therefore sin is being what's right in your own eyes, rather than God's will.
Why? Because each of us are the "other gods". Not that we worship instead of God, but in addition to him! Even devout Christians struggle with self-satisfaction whether it is prideful, sexual, dishonesty or even gluttonous! Each of these things are to magnify or pleasure the self in place of God.
The greatest sin is a sin of omission. We are to love God! We all fail on that ;point because we are to love him immensely, not just perfunctorily. People say "I love Jesus!", but do they really? If they love him so much, why do they continue to disobey. To show our love, we are told to obey his commandments. The second greatest sin is failing to love others. Sure, we may love them, but it's obvious that nearly all love themselves more!
Again the failure is not that we don't love others, but love ourselves more. Witness that with our response when things displease us. Rather than love the other, we protect the self from distress. The self is loved more than others. That is a major sin!
Right this instant if there are those to whom you have antipathy, it's hate that is emoted! It's accidental. You are merely protecting yourself, but accidental sins are still sinful! For many who delight in dissension, that's a sin of omission even though it's willful. It's the inborn nature of people to get even by failure to love!
If by your actions others feelings are hurt or their enthusiasm for righteousness is dampened, it's sinful. They are sins of omission. They are the worst!
For example, blasphemy is the unpardonable sin. It's not what is done, but remains undone. The Holy Spirit calls everyone to salvation, but for those who fail to heed the call, it is sinful. Their antipathy damns them for eternity. It's not an act, but inaction which damns.
We are a sorry lot. We say that we love God, yet hurt other people because they infringe on our own comfort. Us sinners must always be spiritually comfortable. That's what Satan wants us to be. Part of the pleasure he gives is false comfort, whereas it's the Holy Ghost is the real Comforter!
Are you comfortable right now? If you are perhaps you're in Satan's corner. He smiles when we unwittingly stand under his tree! It's not willful, but passive. We think, "It's of no harm!" even as Jesus sheds his tears. You see, the reason we are to abandon sin is to cease the hurt toward Jesus. He has suffered enough on the cross. Why compound the hurt?
What is it that we all need to do? Change! That new person who emerged from the baptismal waters must be changed or Jesus was crucified in vain!