My "focus", gfm, is on the FACT that the reason "sin" exists for Christians is because of the attribute assigned to the god of Christianity...the main one of which, is that damn near everything that humans do, other than adoring that god, offends it.
That, in great part, is the reason today's Christians go to such lengths to disassociate themselves from the god of the Old Testament...THE GOD JESUS WORSHIPED. "The god Jesus worshiped" found many things offensive that modern Christians (and modern people in general), in large part, no longer consider immoral or perverting...and "the god Jesus worshiped" was not offended by many things modern Christians (and modern people in general) deem to be disgusting.
Those who have blindly guessed that a GOD exists may be correct (which necessitates that they may be wrong)...perhaps have bought into notions of the GOD's attributes. To me (this is strictly me), the attributes assigned to the Abrahamic GOD...sound more like the mores and wishes of a relatively unsophisticated, unknowledgeable, superstitious, ancient people...than the dictates of a GOD able to create a universe as immense as ours. Christians may be correct; the ancient Hebrews from whom they derived their GOD may have been correct. And it all may be a flight of fancy...like the flights of fancy of the Greek, Roman and Norse deities.
Sounds good...almost poetic.
The bottom line, though, is that every "sin" is merely something that offends the god. This "what comes first, the taking of offense or the whatever" seems to be opportunistic and gratuitous.
My guess is that Christians assert that everyone is a sinner, because they think it sounds self-effacing...and they consider self-effacement to be something their god demands or likes.