[Beasty]

You absolutely could - the terms are relative and your use of them stems from whatever moral code you've developed as time progresses.

If something isn't good, is it automatically bad?

No

You could have a "bad" day, but is that a "bad" day as opposed a "good" day, or some other kind of day?

...yes?

However, if nothing is good, can anything be "bad"?

Yes. The concept of good can still exist even if bad is all that is applied in the world around you. Nothing being good doesn't necessarily imply that nothing could be good.