I am so sorry this happened. It is so sad for everyone involved. I too agree with everything Glacier said. My dog Duncan will chase and kill anything and everything that runs from him so I have to keep him leashed at all times unless we are in a very very rural setting with noone and nothing around or in a yard. Even in the yard, I stay with him as the neighbor's cats have found their way into our yard on occasion.
I know how horrible you feel...I have been through it myself and it was the worst experience. I couldn't even look at Duncan for weeks afterwards, the memory of it still haunts me.
However, like Glacier said, the dog is doing what dogs do and it is your responsibility to prevent it from happening again. Some dogs just cannot be around smaller animals. I don't know why some dogs can and others can't...? Probably a question for Carrie or Dixie...

Welcome to Pet Talk by the way.