The "good vs. bad" principle is correct but ur perhaps being too simplistic. I mean, how many times do you hv to put your hand on the hot stove to understand how wonderful it felt before you did so.... before you got burned? Only once usually. Right? If you feel the pain of getting burned once, you pretty much know it forever and thus "appreciate" how desirable it is to NOT be in various states of pain and agony.
It's true though as you point out, that as soon as one thing comes into being, there is automatically the existence or at least the potential for existence of its polar opposite. In this way, the world is a kind of constant battleground where each is vying for dominance, sort of.
But that's the point. There doesn't have to be an equal amount of pain to define pleasure, the same amount of day as night, love as hate, good and bad.... In fact, according to the 3rd Law of Thermodynamics, the tendency of the Universe is towards ever increasing entropy, or chaos. I mean this isn't an hypothesis, it's observed everywhere to be true, with no exception, has NEVER been disproved and thus, has the status of scientific LAW.
So don't worry! ALL manner of evil and discord and discontent and death and destruction is out there!! It sure as hell aint goin' away neither! I think the "downside" of all opposites is covered. Thats why it's the easiest thing in the world to be a bad ass, to create trouble and chaos - because you're just going along with the current, floating down the entropic river in your inner tube. While the opposite is true too: It's often the hardest thing in the world to do good, to be fair, to promote harmony in the middle of chaos, to oppose the forces of evil and fight for truth & justice.
This is also why everybody is sorta living on borrowed time. Why there are a million ways of dieing but only one way of coming into this world - and, importantly, why that way is the result of an expression of love between a man an a woman - who are each to the other maybe the highest order of opposites on the planet.
Imo, since true and selfless Love is the highest form of human expression, the ultimate battle taking place day to day, minute to minute is that occurring between love and hate and all the opposites along the love - hate spectrum. Of course I completely agree with Donnie Darko when he criticized his dumb HS teacher who bought into this whole paradigm way too literally in a classroom exercise one day.
But as a general principle, I do think you can understand the most fundamental conflict on this reality plane in those terms. When you consider how everything around us is trending towards higher and higher entropic states, it's easy to see how things are stacked against Love and Goodness and Righteousness and how every moment of the day is truly high drama. It's also easy to see how most people are overwhelmed and often give up, turn bad, take the bribe, go with the flow - seduced by the dark side of the Force. Nevertheless it's also true that Love is the ultimate answer to all conflict. And if it's true that "Love conquers all" - then just think of some of the stunning reversals that could happen if enough people in the world expressed this higher form of love simultaneously and somehow found a way to focus it towards the solving of a certain problem? I definitely think that's possible and that this kind of thing ca become real and commonplace and will represent a new kind of non-material technology that has never before been seen - or perhaps only occurred in ancient times, long before the time of so called "recorded history" - perhaps in undiscovered past civilizations on continents now submerged. Whatevr the history, it's interesting to wonder about this sort of thing anyways.