All things considered, it was probably his fault. When his former master had ascended to his position, it was necessary for the three aspects of his power to separate from him, and take on their own form. Being the Hollow aspect of his power, he had always considered it ironic that he took the form of his former master's "heart". Yet it was only when his Hollow powers had separated from him that his former master truly became Hollow. Perhaps it was the separation, perhaps it was the stresses of his new station, perhaps there had always been something twisted deep within him (he could only hope that it was not the last one). He became more and more twisted, abusing his station for his own ends. He should have insisted on reuniting with his former master - maybe it would have made a difference. But he didn't, and his former master's downward spiral into insanity continued.
It was no surprise that the rebellions began. Yet it was not enough, the rebels were too dispersed, and had no effect on how his former master abused his power.
Then, he met the woman. She was a peasant of no renown or infamy, and yet he chose her to be his queen. Many suspected that it was simple lust, and in the beginning it likely was, yet over time she seemed to soften him. He became kinder, as she showed him how the lowborn and the weak existed, showed him their was more to the world than power. For a time, he was hopeful that his former master could be reformed, that the rebels could be convinced to lay down their arms, that the kingdom could be restored.
Then came the pregnancy.
She had no great amount of reiatsu. He was the Soul King. The fact that she managed to carry the child to term was nothing less than the greatest miracle that had ever occurred in the Soul Society.
Her death shattered him. He reverted to his old ways, worse than before. When he saw that his son was a cripple, he cast him aside, hating him, claiming that he killed the love of his life. With his father unwilling to care for him, Gerard found himself stepping in. He doted on the boy, doing his best to take care of him. When the boy showed his true power, Gerard helped him hone it, taking pride in how the boy used his power to save himself and others. The young prince quickly gained friends and disciples, with many referring to him as a god. Yet this was not enough for the prince, he continued to strive for his father's approval.
The day that the rebels set siege to the Seireitei, the shinigami and quincy quickly found themselves overwhelmed. With the city in flames, and the leaders scattered, the rebel leader ascended to the royal dimension, intent on striking down the Soul King.
When the rebel arrived, only the young prince stood to oppose him. The young prince never chose to speak of what happened next, and all that is known are observations and guesses. They battled at the gate of the royal palace, the force of their reiatsu alone tearing apart the royal dimension. The young prince fought valiantly, yet in the end, the rebel proved to be too powerful, and he forced the young prince to use his then ultimate technique: the Letzt Stil. The young prince tore apart the remainder of the battlefield, and unleashed all of his fury onto the rebel. By the time the Royal Guard managed to return to the Royal Dimension, all that remained of the battle was the powerless, crippled form of the young prince, and the rebel's blade.
Even though the young prince had saved his father, he still found himself rejected. Finally, the young prince had been pushed too far. Conspiring with his caretaker, his tutor, and his closest friends, he planned his escape from the Royal Palace, and then his own rebellion.
And yet, despite everything he had been through, the young prince did not lose sight of kindness. Having taken the name given to him by his followers, the young prince, now Yhwach, sought to use his powers to bless others, and allow them to achieve feats that they could not previously imagine. Using his powers, he finally freed Gerard from his former master's yoke, and blessed him, curing the ache that was brought on by Gerard's hollow nature. This was why, now, over a thousand years later, Gerard continued to follow him; to provide the boy he considered to be his son with guidance, and to see to it that the boy would dethrone his father, and bring his blessings to all of creation.