I think it takes a bit too much rationalization to imagine a "good Leah" scenario. I don't think it fits. She was a murderous psychopath from the start and there are several suggestions that it isn't her first rodeo either. Her musings about "the last one" as well as Jessie's observation that Leah was extremely "self-aware" both suggest that she had done these things before. Leah was pulling off extremely risky stunts, including burgling a home multiple times with the occupants in it, and raping coma patients in her own hospital. She did these things without even the barest hint of insecurity or worry of getting caught. In other words, that suggests she knew how
not to get caught. Leah is like a
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, but with really great tits.
Jonas didn't even figure out what she was up to until the moment
she let him figure it out. The moment she realized he'd never love her, she decided to kill him... So she left Megan's bag and shoes outside the hallway as breadcrumbs to lead him the secret room while she was changing her hair and dress. She was in complete control until she fucked up.
Even if we were to imagine a Leah ending scenario where Jonas remained faithful to her,
she will still kill Liam. She already decided to kill Liam while Jonas was in a coma which means she would also presumably kill any one who she feels threatened by or any one else that might get "too close" to Jonas. Then she'd probably kill Jonas if she ever gave herself a reason to. I mean consider that she literally decided to
kill Jonas's best friend for something as minor as making a comment about Jonas banging bitches. This is like Hannibal Lecter killing the tuba player for being a little flat in the orchestra. I don't think there is ever meant to be a happy ending with that.
Leah was a always False LI from the very beginning. That's what makes her character so neat because most AVN devs only consider "plot" to be an excuse to get from one sex scene to another. I feel like if there were an ending for a character like her, it would feel too contrived.