Some female mice are just very territorial, same with rats. And they will also eat the others if they feel threatened by the others. It happens alot, really it is common. I have seen mice do this alot. Rats are not as bad, they will fight to the death , but it happens alot less with them. All you can do is choose not to breed that one anymore, or make sure you separate it right away from the others. They do have personality conflicts like us. As far as breeding rats, that is up to you. It has been said that rats are supposeadly better for your snakes, they have more calcium and they make the snake grow faster when being fed rats. I have had snakes for a long time, and to be honest, I have not seen much of a difference between a snake fed mice or rats. Unless ofcourse, you are power feeding, and you still will get the same result no matter what it is you are feeding your snake. The rule of thumb in feeding any snake, only feed it prey as big around as the thickest part of its body. So, really I think it is personal preference on this one. And I know that there are others out there who will disagree with what I have said... so, once again, I state THIS IS PERSONAL PREFERENCE. I have over 14 years of experience with different kinds of snakes, but I am in no way an expert, what I know is based on personal experience. And trying different things with my snakes to see what works.