A landlord is not responsible for the tortious (something you could sue over) or criminal activities of other tenants, their families, or their guests--and certainly not of other people who are non-tenants (if the neighor is not a tenant of this landlord). The fact that the neighbor's boyfriend posed a threat and was arrested is simply not the landlord's responsibility--unless the landlord is the neighbor to which you are referring--and so the boyfriend's action does not provide grounds for a rent reduction or rebate, or to terminate your lease without penalty  except as is set out  in the lease itself (i.e. by providing proper notice and paying any amounts due).