"Because of intention, the essential nature of sin was seen as a deliberate and willful choice to harm the relationship." That's the core, right there. Sin is not so much activity, but the intention behind it. The intention is what causes the action.
I've heard some people define sin in such a way that humanity was sinful even before the fall! I wonder about their understanding of the incarnation in those cases. If existing as a physical being is sinful, then the incarnation didn't really happen—they call the docetism (Jesus just appears to be human, from Greek δοκέω/dokew, to seem) and the church denounced it as a heresy many a long year ago...