Ok, so if light side is inherently balance, the contradiction would be that the users of the force are all sentient life forms that are going to tend towards personal gain, and thus tend towards the dark side. "Human" nature is the counterbalance to the force that so often leads to the dark side of the force.
The light side of the force is a state of harmony and balance, and the jedi strive towards that path, but just their mere existence as sentient individual lifeforms first, and jedi afterwards, contradicts with the path they want to follow.
This is why Anakin being older was more important: he had more time on his own.
Why was the old Jedi Order messed up, and need of balancing? I suppose, the more individuals, both jedi and sith, there are, the more variables that can work towards personal gain, whether it be minor or major in its severity. But to counteract individual desire, "humans" inevitably create institutions and attempt to get closer towards collective thought, however the very action of creating institutions that need to be so large that they span the galaxy and can identify any and all force-sensitive people would require more individuals which creates that instability. The old Jedi Order being so large would be the issue, and this was less a situation of a presence of sith that was an issue so much as the sum of all the minor individual desires among the Jedi.
Thus Anakin kills them all off to get rid of that sum total, then kills himself and Palpatine, as Sith inherently screw up the balance, and Luke tries to reestablish a new Jedi Order that can grow and locate force-sensitive beings across the galaxy so as to train them away from pursuit of their own selfish desires, etc. etc.
Mind you, I'm not that big on Star Wars, I'm just taking the given situation and expanding on it. Lucas essentially gets an infinite number of cycles and stories to work with, and the story is just about human nature, like many other stories, special counteracting galactic sentience called the "Force" be damned.