how about this
If you have the ability to do something good and do not do it, is that a sin?
if you have the ability to do the 'best thing' or 'most good thing' [assuming that there is always one best thing] is it sinful not to do it?
if so, then if we are without sin, won't we always do the 'best thing' hence becoming mindless 'do best thing' drones? Will we still have free will?