good question!
now why would the huge mouse have a baseball bat in the first place?
was said huge mouse going to a baseball match, or was the mouse weaponizing the baseball bat for self protection?
perhaps the huge mouse found the baseball bat and was returning it to an as yet unknown entity?
then again, as you intimated, the huge mouse was not in fact in possession of the purported bat, which was, in fact, allegedly in the hands of the unidentified OP who then whacked the huge mouse and killed it
we may never know the true story