This is complicated and thorny from a legal perspective. The obligation you have is (primarily) to your landlord, not to co-tenants. In most all circumstances, a co-tenant cannot unilaterally kick your out or otherwise evict you. If you have legitimate health and safety concerns from co-tenants or guests of other tenants, though, you do have additional rights (with respect to the landlord) which may include terminating your lease. Privately, you may have other remedies available against the threatening individual personally, including seeking a restraining order or the like.
Answered on Jun 22nd, 2012 at 2:06 PM