But ultimately, all sin is sin against God. Think of Joseph when confronted with Potiphar's wife panting to take him to bed. This was sin against Potiphar, and Joseph knew that. But he knew this was first sin against God. Or, think of King David after his adultery with Bathsheba and his murder of Uriah.
Sin is something man is born with.
It is something all men inherit. It is something that corrupts from within so that words, thoughts, and deeds are all infected. To put it plainly, Jesus brings salvation. He brings salvation from sin. You all know how He does this.