Mom Rat Got Mad?

I went to check on my rat babies this morning (a week ago i put a rat pup that was a few days older then the rest in with the mom, and she raised her up until she opened her eyes..) and the rat pup that was a few days older was gone! So i dug threw the bedding and found it with it head crushed open, and brains coming out! What could have caused this? Should i have weaned this rat earlier? She didnt touch any of the other babies.... The other babies eyes are still closed but look like they'll open soon...

If it was a runt orphan, more than likely she was too far behind to make it. Runts, even after making for a week or more, often die on their own. Could be from underdeveloped organs due to their runt status, or other reasons.

Don't beat up mom, she more than likely just took advantage of the free nutrients. Remember, rats eat meat too!