What difficulty that doesn't persist in religion (except you choose one specific, but even then it doesn't fully help) is there to define good or bad (apart from the difficulty to define anything like what is a 'sheep' for example).
On the other hand, the problem of what’s good and what’s bad is of course far from being settled, at least looking at the diversity of opinions people have about the matter.
It’s pretty much the definition of value system, actually!