Your niece bought two more from the same parents as the one she put down?
That's terrifying.
Mine is predatory towards animals and would kill every cat she had the ability to come into contact with. If she saw a sleeping cat, she would still get it. It's my job to keep that from happening.
People have cats running loose and have no problem with them killing other animals. They don't freak out and have the cat put down because "what if that bird had been a child?!"
Dogs killing animals is not in any way tied to human aggression. All terrier breeds are notorious for loving to kill other animals. That's what they were created for and is to be expected AND managed responsibly.
Now, if the dog is also sketchy/fearful/shy/aggressive towards people, that is a serious problem. I would never go back to the same place and get TWO more.

Predatory towards other animals but not small humans
You're an accident waiting to happen.