I'm not depending on God or anyone else to mete out justice in my favor, I'm simply concerned with how I ought to act so as to be just and charitable to those I meet.  Assuming the best of people, when possible, is a very good way to achieve that end.

Regarding sin, there really aren't that many acts that would be wrong by their nature.  Look at the Ten Commandments, that's about all of them.  Pretty much everything else we can conceive of doing is generally good, but may be made bad in certain circumstances.

Easy example: there is no case where theft is morally acceptable.  Now work and earning goods, on the other hand?  Generally good, though circumstances or intent may perhaps render it bad at times.