Legal Question in Landlord & Tenant Law in Massachusetts

A landlord has an obligation to provide a unit fit for habitability at the time of the tenancy, and an obligation to reasonably maintain the property. That said, a lease is a contract between landlord and tenant, which the tenant may breach by failing to pay rent (or by destroying the property). Have you orally agreed to a lease with the landlord? A lease term? Have you given the landlord notice of the mice? What evidence do you have that makes you believe there are mice in the apartment?

Simply moving out because you do not like the apartment, however, might lead to the landlord trying to extract the cost of rent either until he can re-let the place, or until the end of the term of your agreed upon tenancy. Of course, he would have to prove the lease and the lease term... Moreover, he might just re-rent the apartment, because suing you would not be worth it. It is impossible to say without knowing more facts.