Female rat killed her sister

Question
Last night, my friend discovered that her rat Thisbe killed her sister rat Cait. She, in fact, decapitated her. Thisbe has been the dominant rat but she has never injured Cait. I have owned many rats in the past and have never had one kill the other. She kept them in a 10 gallon aquarium and I suggested that she get a cage because rats prefer it. I am completely baffled as to what happened and she is very distraught. Any insight would help. Thank you.

Answer
Hi Jenna

I am almost 100% sure that, unless someone witnessed this, the Cait had already passed from unknown causes and Thisbe was simply doing what many rats do, which is dispose of the body by cannabalism. Although a 10 gal tank is a poor choice for living quarters for rats, I doubt it drove Thisbe "mad" to the point she would do what she is suspected of doing.
Rats in the wild dispose of their deceased by consuming them in order to hide any evidence of their prescence from predators. This is not only to protect the colony by a sign of respect as if to not want any predators to have the deceased either, so they consume them. Some pet rats continue to carry this trait of their wild rat ancestors and others may simply opt to bury them, which I have also seen happen.

Did anyone see Thisbe actually beat up and kill Cait or was she just caught in the act, or was Cait found this way?

I am really sorry this happened though, esp for someone to see this with their beloved rats. It makes you just wonder what on earth went wrong, but again, chances are, Cait died on her own first.  How old are the girls?