Depends on the circumstances and how old she is/where she is in life! I think Isabelle still living with Alec, still under his thumb, would be so horrified with herself and so bereft of love and purpose in her life that she would become suicidal, and it would probably take a lot of external effort to keep her alive. Alec brainwashed her into thinking he was the only thing redeeming her existence, after all.

However, I can see another scenario where Isabelle is older and kills Alec by accident in self-defense when Alec tries to bring her back to Aldwycke after she escapes. In that case, while she'd be extremely distraught and angry with herself and would struggle to function enough to flee for her own safety, I could see her recovering more readily since she's already done some growth and gained some independence.