If you break the lease without establishing cause, you may be liable for the remainder of the lease payments, but Massachusetts law requires the landlord to make a good faith effort to find a replacement tenant.  So, no, the landlord cannot just sit back and collect rent from you after you leave.  Also, if you stay, your landlord has an obligation to maintain the premises in a way that does not interfere with your quiet enjoyment.  He or she can also evict tenants under an expedited eviction process if they are committing certain specified crimes within the unit.   In this situation, your best and cheapest solution is to talk to your landlord and hope he or she will work with you to help resolve the problem (get rid of the other tenants or work out an exit strategy for yourselves).

If your landlord will not cooperate and you feel (for safety or other reasons) that you have to leave, then you should.  Procedurally, there are probably a couple of ways you can do this.  I would contact the housing specialist at your local Housing Court and ask for some guidance.  You might bring an action on your own seeking an injunction requiring action on the landlord's part, for example, or allowing you to break the lease.  Or you could take a more passive role.  Document the problems first in writing, collect any evidence you can to support your claims (do not record conversations without the other parties permission), ask the landlord in writing to remedy the situation, then go. If the landlord later attempts to collect the rent at court, make your arguments regarding quiet enjoyment.  Good luck.  For more information about these issues, visit our Massachusetts Tenant Law Discussion Forum.