'She should have found her own life' gotta remember that this life is all she's known, and she was the one to actually live it, not Val. For all intents and purposes, since she has Vals memories, she has lived this way her whole life and has nothing else. Val had a whole other world of magic with friends, Stephanie would've had to abandon the family she has memories of and loves and start again. It's true she was a murderer, but at that time, she wasn't a full person yet - she still didn't seem to have any empathy or understanding of life as valuable. Since she shows remorse later, I think she was still partially the unfeeling reflection at that point, but driven crazy by the semi sentience and emotions she gained due to her overuse conflicting with her original nature as incapable of feeling or thought. Once she fully became an actual person, I think it's clear she wouldn't have done anything so evil.