‘Sin’ is perhaps the wrong term. Sin as theists take it is doing wrong against God, against others, and against the self. Obviously an atheist (or an agnostic for that matter) would not consider the first of the three an issue at all.
Another thing to keep in mind is that atheism and agnosticism are not religions; they’re positions on a single theological issue. Trying to ask if atheists
in general believe thus-and-such (other than ‘does God exist?’) or prefer boxers/boyshorts to briefs/grannypanties will get you nowhere. It’s like herding cats – although I’d be interested to see the results of a poll on the latter question
That said, a good many of us do in fact admit to the reality of wrongdoing, or ‘sin’ in the latter two senses. Some of us will even go so far as to say that certain things are absolutely, universally right and wrong! Not that that will necessarily stop us from wrongdoing out of convenience or necessity now and then (as in the classic Gestapo-at-the-door scenario), but if you were better than we in that regard you wouldn’t have the sacrament of confession, now would you?
Atonement varies even for theists, as I’m sure you know well. One priest may tell you to say three Hail Marys, another may have you go a rosary or two, another may tell you, as in my childhood, to keep your room clean for a week (
thanks, Father Chuck…grumblegrumble), or, as the same priest to my mother, to bake cookies for your kids (thank YOU, Father Chuck!).
Personally, I figure that wrongdoing is absolute, but atonement is subjective. Ideally, it’d be enough that the penitent and the victim could agree that they are once more on even ground, and choose to continue to interact or just to go their separate ways. There’s absolutely no way to quantify just how much would be needed speaking universally for a particular instance of wrongdoing: one person may not be as adversely affected by the wrong as another, and so the degree of atonement is different.