Surprisingly Fun
There were two main reasons why Rotbart Von Schwarz, son of the wicked sorcerer Eric Von Schwarz, had elected to take Raven Queen's place as the villain in the Snow White story after she had rejected her destiny.
One, he was Raven's big brother. Well, half-brother anyway. Not many people knew that the Evil Queen had had a passionate, tempestuous affair with the evil sorcerer of Swan Lake fame before marrying the Good King, Raven's dad. Nobody could guess it from looking at the two siblings. While Raven closely resembled their beautiful and infamous mother, she exuded humility and compassion, which she had learned from her father. Rotbart on the other hand, was undeniably evil (or at the very least, extremely dark). And incredibly powerful since he was the son of not one, but two, great sorcerers. His flaming red hair, confident stride, and the glittering glint in his eyes said to anyone that crossed his path: I could totally fuck you up if I wanted to. Nobody ever got that impression from encountering Raven.
Despite their immense differences, the two siblings had a strange affection for each other. Even though Rotbart mainly lived with his dad, he would often come to the Good King's castle to hang out with his mom and little sister. For as long as Rotbart and Raven could remember, the other one had always been hanging around (since they were only a couple years apart in age). And now they were all they had left of their mother, who, despite everything she had done, was their Mom above all. In brief, they were both lucky enough to feel the indelible love that comes from sharing a parent and a childhood.
So when Raven decided that she didn't want to even try to be evil, her big brother quickly offered to shoulder her burden for her, out of that indelible love.
The second big reason Rotbart decided to abandon his original destiny in favor of Raven's was because he loved Duchess Swan. When Duchess Swan angrily confronted him about leaving their story, he told her the truth: he would rather suffer in another story than make her suffer at all. He wanted her to be free and to find love and happiness, even if it wasn't with him. Granted, Duchess didn't react to his selfless act quite as he had hoped (she had been extremely angry and fearful at the time), but he could take comfort in the fact that she was now free to pursue a romantic relationship with Sparrow Hood (even though Rotbart personally felt that Hood was an annoying little prick).
But not too long after meeting Apple White, Rothbart realized that he had another excellent reason for switching stories:
Being in Apple White's story was actually fun.