What is sin? Often we hear sin described in the manner of deeds. I.e. "Sin is doing something God told you not to do, or not doing something he told you to do." However, Biblically it is apparent that sin is something more, something that cuts through the hearts of everyone.
Lucifer demonstrates this sin in his rebellion against God. He thought that by his might and pride, he could overpower God. The thing he didn't realize is that God is Most Holy and pride makes one less powerful than Him. Thus by violating the humility inherent in the nature of God, Lucifer became even less powerful and holy.
Likewise, we must realize that sin is not sin just because it violates some independent rules of ethics. Sin is a violation of the very nature of God.
The question is often asked in ethics, "Is Right right because God said it was, or did God say what was right based on what is Right?"
However, as with many such dilemmas, the problem is in the question. Right and wrong are not determined by a declaration of God but by who God is. God is Right; therefore, righteousness is anything that is in line with his nature.
When we sin, we violate the nature of who God is. It's not a matter of breaking or keeping rules but breaking or keeping the relationship with God who is Holy.
(For those concerned about our futile human efforts to be holy, see this recent post.)