In my experience (I produce several thousands a year) rats rarely fight to the death unless you have males house together that did not grow up together.
Now can it happen yes but again RARE, lack of food, improper food, lack of water could lead to such issues.
Now you also need to take in consideration that commotion when introducing a new rat is natural, but it does not mean they fought to death either.
Your rat could have died of natural causes and since rats are scavengers they clean up and eat their dead.
At this point you really do not know what happened.
The key with rat breeding is proper nutrition and selective breeding (eliminate any overly aggressive rat from your breeders.)
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