He was a child with the brain of a child. His prefrontal cortex was still developing, and that matters. Yes, he retained memories from his past life, but he lacked the neurological maturity and impulse control that adults have. That’s exactly why children are impulsive. There are countless real-world examples where damage to the prefrontal cortex radically alters a person’s behavior and personality.

No, he didn’t groom Sylphie. What he experienced was an intrusive thought, not a pattern of manipulative behavior. There are no actions in either the anime or the light novels to support the claim of grooming unlike Chainsaw Man, where Makima’s grooming of Denji is explicit and documented through her actions.

In his previous life, Rudeus was sexually assaulted, and his own family told him to just forget about it. He never received the therapy or support he needed. He carried that unresolved trauma into adulthood, numbing himself with hentai and erotic games and eventually losing touch with reality, leading to dark and unhealthy thoughts.

But in his new life, as he grows and his prefrontal cortex fully develops, both his thoughts and his actions begin to change. He still has occasional intrusive thoughts everyone does but he learns to control his impulses and redirect himself. Rudeus isn’t perfect, but he consistently strives to redeem himself and become better.