Dog killing another dog?

I have a 2 year old, 35 lb, dog I rescued from Kuwait. He was abused and has many fear/aggression issues. I have been working everyday with him for the last 2 years. I exersice him daily, I have gotten him accupuncture, used herbal calmers, dog prozac, dog training, and recently I have consulted with a dog behaviorist. 3 days ago while on a family vacation he killed my parents 3 lb dog without cause. I am now worried for the safety of my other dog (a Min Pin) since he has also attacked my parents other 2 dogs. My dog behaviorist told me to have him euthinized. I don't see rehomeing him as an option. Is there help left for him?