One of the frustrating things about aggressive dogs is that while the law is quite strong (in word, if not always in actions of individual members of the legal system) when it comes to dog attacks on humans or livestock, it’s very weak when it comes to dog attacks on other dogs. This is a nightmare for dog owners because if a dog attacks you and your leashed dog, the chances are good he/she will go after your dog first - it’s smaller, less intimidating, and usually closer - which will muddy the question of whether Cujo is a danger to humans.
In short, I think it’s dangerous to assume that the law would be on your side if you hurt or killed this dog in self-defense. Don’t get me wrong, I’d rather face possible cruelty charges if the only alternative was standing by while my dog was killed - but I’d rather avoid the situation altogether. If you think the mess is due to move on soon, do what someone else said and relocate for walks. Sucks, I know - I did it for several years in a similar situation. On the bright side, I did discover several great new places to walk the dog!