The answer to the question in the title is that you should praise your cat for her maternal generosity. You should then take the mouse from her while stroking her and complimenting her. You should then quietly dispose of the mouse if it is dead. If it is alive you should place it outside in a place where you think there is a mouse nest which is where your cat is likely to have caught it. That, I think, is the best you can do. It reflects the attitude of your cat because on the occasions that they bring a mouse into your home for you, they are acting as if you are a member of their family, a kitten. If the mouse is alive, they are bringing it to you to show you how to kill it as part of their maternal training leading to your independence. If the mouse is dead it is to feed on it.
Under natural conditions i.e. not living in a human environment, cats introduce prey to their kittens progressively. At first when they bring the prey back it is killed and where it is caught and eats it while her kittens watch. The next phase is to bring a dead prey animal back and play with it before consuming it. The kittens see her batting the prey with her claws and grabbing in her jaws.
The next step is to leave the dead prey to be eaten by her kittens. At this stage the prey brought in is dead in order to protect her kittens. When they are older she will kill the prey in front of the kittens to teach them how to do it. They watch and learn and then they accompany her on a hunt and try killing prey animals for themselves.
Both females and males bring mice back to the human home; the cat’s den. Males are far less involved in the upbringing of their offspring and therefore what I’ve said above appears inapplicable. However, males do get involved sometimes in raising their kittens and I would suggest that what I have said applies both to males and females equally.
If a cat owner criticises their cat for bringing a mouse to them they will be nonplussed. It would be another example of a lack of communication between cat and human caretaker. If the cat is shouted at or in some other way intimidated they will find their treatment incomprehensible.
What is happening when they bring prey back is a reversal of roles to the usual state of affairs. The usual relationship is that the human caregiver is the surrogate mother and the adult cat is the kitten. You can see that domestic cats can become confused as to their role in the human environment. It’s also partly why you sometimes see cats carrying smelly socks around as if they are carrying kittens. Cat sometimes “steal” items of clothing from a neighbour as if bringing a prey animal back to their den i.e. your home. There is this, I would argue, state of confusion which can sometimes pop up in the mind of domestic cats. They are a smidgen away from the wild doing their best to adapt to the human environment. This creates this mental on-off switch between domesticated and wild behaviour.
