Potentially! I've had AUs where she's killed one or both of her parents. My lovely friend Ira wrote a fic where she and Alec kill Viola together, and it's entirely possible for her to snap and kill either of them. Killing Viola would twist and scar her; killing Alec would, under most circumstances, shatter her. But it really depends. I've always known that if Isabelle had children of her own, she'd defend them from Alec as far as she possibly could, and that could include murder.