The Primordial Beasts of Sin were fine when they were doing their passive duty of absorbing the negative energy which they call Sin Energy. Once they collect enough, the Primordial Beasts of Sin would purify the energy and release it to the world to become nourishment. This is a way for the world to maintain itself even with negative energy. This was the benefit that the world was given when the Primordial Beasts of Sin worked for the betterment of others.
It was only when the Primordial Beast of Sloth got the visions of the past that the Primordial Beasts of Sin have changed. All of them decided to thirst for power as if something inside of them was triggered. The Primordial Beasts of Sin committed the taboo of actually fusing with each other to regain some of the power that they lost.
