We would still know what bad is if we lived in a good world.
Because even though you are good there is probably some one thats even more good than you so you would consider yourself bad.
the same goes with bad. You may be bad but the guy next to you may be evil.
So anyone on the other planet got no chance of being Good while on the other hand anyone on our planet got no chance of being Bad? dude, you're demolishing the entire base on which humanity stands. See, we are what we are cause we are capable of doing both Good & bad and its only up to us to choose which to do and which to fight back.