I am so sorry to hear this tragic story, my best wishes to you and your fiance and little Commodus. Commodus had loving owners and was victim of an accident. It wasn't the dogs fault it was just doing what it thinks is natural, Jack Russells hunt rabbits and will chase anything small and furry.
It was an accident, you must not blame yourself, it was unfortunate, I know no words will help with the pain but we are thinking of you and Commodus right now.
I feel sorry for the lady with the dogs too, as it wasn't the dogs fault.

A cat we used to feed was mauled outside our house by a dog that walked past with it's owner, but I don't blame the dog, I blame the owner for not having it on a lead. So try not to blame the dog, but I know as a true animal lover you wouldn't, it didn't understand what it was doing.

I'm so sorry.