Sin is also a bludgeon that Christians use against non-believers -- "We're saved and you're not, neener neener neener!"
Not even remotely true. It's a very sad thing when a person is not saved. Nothing to gloat about whatsoever.
This isn't a game. This is serious. Souls are at stake.
Plus if you don't believe in sin, one might be inclined to go out and kill somebody.