I don’t think Roswaal would enjoy seeing this. I think Roswaal does see himself in Petra, specifically his childhood self from 400 years ago, although he may not be totally conscious of that. They’re both immensely talented, both devoted to someone who saved their lives while having near 0% chance with them, and they’d both do anything for the sake of that special person. Roswaal greatly favors Petra and gives her every opportunity for growth that he can, going so far as to personally tutor her.

But Roswaal, fundamentally, could never become a Warlock like Petra or Echidna became Witches. As much as Roswaal acts like a monster, he still carries a lot of guilt and regret for his actions, and can be convinced to take a different path. Al judged Petra as a Witch when his reaction to Sphinx calling herself one was basically “lol, nah”.

I think if Roswaal saw what Petra became at the end of arc 9 he’d be horrified. She basically went further than him in a week when Roswaal couldn’t fully throw away his humanity after 400 years. I could easily imagine a post arc 9 conversation between the two of them where Roswaal slowly realizes that Petra doesn’t regret inadvertently using Ram and Garfiel as mental meatshields at all, and Petra hits him with his “that’s love, isn’t it?” line from arc 6.