De acordo com a novela oficial da Disney Canon por Alan Dean Foster, nós sabemos duas coisas:
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Han e Leia suspeitaram que Ben sempre teve um puxão para um lado negro, como Anakin. Essa foi uma razão pela qual ele foi enviado para estudar sob o comando de Lucas
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No entanto, um palpite por @ user3574984 em outra resposta está errado - ele não apenas caiu para o Dark Side sozinho, e então saiu e se conectou com Snoke. Snoke teve parte integral em seduzi-lo para o Lado Negro - embora neste momento, não tenhamos informações canônicas de qualquer nível que apontem para detalhes de "como".
What could he say to that? What possible response could he give to a willful denial of reason? “He’s gone, Leia. He was always drawn to the dark side. There was nothing we could’ve done to stop it, no matter how hard we tried.” His final words were the hardest to get out. “There was too much Vader in him.”
“That’s why I wanted him to train with Luke,” Leia said. “I just never should have sent him away. That’s when I lost him. When I lost you both.”
Han dipped his head. “We both had to deal with it in our own way.” He shrugged. “I went back to the only thing I was ever good at.”
“We both did,” Leia admitted.
He met her eyes steadily. “We’ve lost our son, forever.”
Leia bit her lower lip, refusing to concede. “No. It was Snoke.”
Han drew back slightly. “Snoke?”
She nodded. “He knew our child would be strong with the Force. That he was born with equal potential for good or evil.”
“You knew this from the beginning? Why didn’t you tell me?”
She sighed. “Many reasons. I was hoping that I was wrong, that it wasn’t true. I hoped I could sway him, turn him away from the dark side, without having to involve you.” A small smile appeared. “You had—you have—wonderful qualities, Han, but patience and understanding were never among them. I was afraid that your reactions would only drive him farther to the dark side. I thought I could shield him from Snoke’s influence and you from what was happening.” Her voice dropped. “It’s clear now that I was wrong. Whether your involvement would have made a difference, we’ll never know.”
He had trouble believing what he was hearing. “So Snoke was watching our son.”
“Always,” she told him. “From the shadows, in the beginning, even before I realized what was happening, he was manipulating everything, pulling our son toward the dark side.”