Though they are terriers so small animals running may turn their chase and kill small rodents instinct on. If they've attacked and killed two dogs though they need to be quarantined and wear muzzles when they are outdoors. Then they need to be evaluated by an animal behavior expert and be trained, and then never be allowed outside unsupervised.

This is exactly why people need to keep their animals indoors or supervised while outside though. Bored animals will get into trouble. Our animals rarely ever got out when we didn't know it. Only time was when the beagle's chain broke so she wandered off. People just are not responsible for their dogs.