Violation of any command is an act of disobedience or rebellion. Sin entered the world through the disobedience of one man. There are many commands from God unambiguously expressed in different parts of the Bible. Contravention of these commands is a sin punishable in different ways.
The most rtragic consequence of disobedience, however, is the hardening of heart due to habitual practice, which makes it impossible for the guilty to confess his/her sins and seek divine forgiveness. The sin of disobedience thus separates us from the glorious presence of God, an eventuality played out in the very early days of God-human interaction.
Another point to be noted is that there is very little leeway for us to justify ourselves by means of contextual explanations of our disobedience. Once we are convinced of our error, the only way back to His grace is through confession and renunciation. Remember the Pharisee and the tax collector at the synagogue, and the contrasting cases of the two thieves on the crosses flanking our Lord's cross on Calvary.