What to do with a vicious mouse...

I usually always put my young does in with my elderly does... so that's what I did. However, Aylithe (a year old doe that I have in this cage because I never want her bred) has become vicious and it keeps getting worse. This is the first time she has ever visibly wounded another mouse..... She has nearly killed Nemo. Nemo's back end is stripped of hair and bloodly. Her back leg injured to the point where she can barely move...

I'm debating if I should just put Aylithe down.. I was once so close to her...but this is like a totally different mouse.

It sounds more serious than a dominance fight.
I would cull. Aggression in rodents is heritable and you don't want to introduce that behavior into your lines.

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Yeah, that's the reason why I decided not to breed her and it was a good call since now she is worse.