Unfortunately, stray dog packs that enters zoo ground and kill its animals is not so rare even in Europe, we had similar case in Prague few years ago. A pack of unsupervised dogs who were owned by people living in nearby anarchist squat had jumped the fence early morning and killed many pygmy sheep and goats in a petting zoo.
However I don´t agree with the assumption such dogs are more prone to attack people. Pray drive is normal in many dog breeds and nothing to worry about (as long as owners have good control of their dogs). No need to call for authanasia. Just let the owners pay the damages and fines, investigate their circumstances and way they work with the dogs and if you find it is prone to happen again, just seize the dogs. They should be adopted by better owners.