Any dog can be a killer. My parent's Jack Russell, who is a very spoilt boy, is a good example. He is a fair mouser. He's caught a bird, just one of those nasty introduced Indian Mynas, and one New Years Eve he bagged himself a possum.
Mum and Dad hosted a party in the backyard that year. At the end of the evening they decided to leave the cleaning till the morning. In the morning Mum got up, went outside and found grey fur all over the backyard. After a little consideration she realised that a possum got into some of the wine left in the glasses on the table. The possum got drunk and so the dog got the possum. Poor thing. I think it took Mum at least a week to get over it she felt so guilty, but how could she know that would happen?
Anyhow, any dog can kill. Even the most pampered pets.
Edit: I agree Laura77 Dogs trained to kill/hunt are a different thing, but at the same time it must come back to how the owners treat them.