It seems to me that if you believe in free will and that we can be held accountable for our actions, then you have to believe in sin, at least by a broad definition.

An excellent piece of advice that I try (and often fail) to live by is to always assume virtue of other people if at all possible.  Usually that's way harder than assuming vice, but if you assume the best of people by default, it's way easier to treat them generously.