You cannot change a dog's nature. I do not believe that Cali should be put down for doing what is natural to her. You just have to make sure she is NEVER around other animals. IF you cannot keep them separated full time so Cali cannot attack Nala then the only option is to give Nala up. If you love Nala and feel bonded to her then why do you want to keep her where she will keep being hurt? What happens when you finally aren't able to separate them in time? Part of loving someone is doing what is right and best for them, even if it's not what you want to do.

You have learned a valuable lesson: no other pets while you have Cali. MY heart truly goes out to you. It's a horrible position to be in. Please make sure that your yard is escape proof so Cali cannot get out and attack a neighbor's pet. That happened to a good friend of mine. Her pit was a wonderful family dog. One of the sweetest dogs I've ever been around but she was very animal aggressive. She got out and maimed the neighbor's german shepherd. My friend had to pay over $23,000 in court costs/fines/vet bills. Please be careful.