If they had the Three Rules, thn they would stop any kind of killing of a human. If any rules are broken, then they die. The rules leave no loop-holes for killing a human.

1. A robot may not injure a human being or, through inaction, allow a human being to come to harm.

2. A robot must obey any orders given to it by human beings, except where such orders would conflict with the First Law.

3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Law.