Good and bad both exist because we want to go somewhere, or we want a certain development to happen. Therefore, the development that we want to see happen is "good" while everything that stands in its way is "bad". Yes, it is that simple, assuming you have a clear picture of where you want to go.
But not everyone wants to go to the same place, or reach the same way of living, so it's more complicated.
Oh and i forgot to mention: If the world existed in perfect balance, with nobody ever wanting to change anything, neither good nor bad would exist in the world. The world would just be.