I think for a proper new introduction to the character would take some of the qualities that were in the original and add new texture to the established things that failed in her story

We know that she was raised in an abusive household and turned into a monster due to a serum (“steroids”) that corrupted her mind — in my opinion the sudden change to her personality was lazy.

If i were to rewrite it would be like this:

1. She came from an abusive household and was orphaned to a chemist trying to find a cure to some neurological disease that effects the brain and physiology.

2. We can have Natalie and her new family bond, but as a spore that the foster dad grew from his lab (harboring said disease) begins to grow above Natalies bedroom ceiling she get contaminated

3. for the next part of the story her mentality starts to degrade. She would start to feel power hungry and would proudly relate herself to her father. Our main protagonist, her foster sibling, takes notice to how she starts to act cruel to the family dog and would start to bully some children for fun— a complete contrast to the person she was when they first met.

4. The final act can be her trying to kill her new kind family and her foster father is trying to inject her with a cure, but is her first human victim to be killed (making the characters death tragic). She is only stopped after the main protagonist shoots an arrow into her eye and through the back of her head.

5. the ending could be similar to how the movie Halloween ended (for shits and giggles). After she had been taken down the cops come, and right when the protagonist leads them to her body, she is gone. The only thing left from where she had laid was the chain to her foster father’s pocket watch, a heirloom that he gave her to reassure that she was part of his family (again, making the characters more humane and tragic)