One rat killing another one.

Question
I am a first time "mom" to a young female rat, still small and but growing fast.  In reading up on rats I see where they like to have a companion. I got another female, which was younger and smaller. They seemed to be fine after the introduction but the bigger one killed the smaller one! I am so upset and I need to know why this might of happened. What should I do now! I feel so bad. Thanks.

Answer
This is not common at all.  How long did you have the older rat before you brought the new one in and how long did you proceed with introductions?
I am so sorry this happened, but know that it is not common at all for female rats to just up and kill another rat.  Did you witness it?

Could the baby have died on her own and you found the older rat trying to get rid of her body?