You need a lawyer and you need to go through the legal system.
Mitchell S. Feldman
Associate Broker/ Director of Sales
Madison Estates & Properties, Inc.
Office: (718) 645-1665/ Cell: (917) 805-0783
You do need a lawyer. If you do not get a lawyer, your unsavory roommate might beat you to the punch and get a lawyer themselves.
A friend of mine had a similar situation a few years back when he moved into someone elses apartment. Things got really bad to the point where they got physical. Because my friend got a lawyer first, he ended up getting a settlement and his roommate who was the lease holder had to kove out of his own apartment for 3 months to give my friend ample time to find another place. All of this rent free!
Get a lawyer and definitely talk to your landlord. If your roommate is not on the lease and you are, he might be able to help you get the roomate evicted before the situation escalates any further.