This question is probably more appropriate for the lawsuits section, but I will take a shot at answering it. In order to have a viable claim for a lawsuit, there must be some kind of damages that the plaintiff has sustained. Damages could be emotional damages, but that type of claim (called intentional infliction of emotional distress) is very hard to win and the emotional distress must be severe. I think that the real question is if there is a way for the roommate to be removed from the premises. If you feel in danger for your health or safety, then the landlord may have a reason to evict that tenant. However, before you talk to the landlord, I would speak with an attorney to go into more detail. There are obviously many factors that need to be considered.
Is this roommate on the lease? If not, just kick him out. If he won't go, call the police. He is a trespasser. If he is on the lease, it is trickier. The landlord won't help you because it is not his problem as long as the rent is paid. If the guy is acting up enough to get the cops called and catch a charge, he might care then, but landlords usually don't get involved in disputes between roommates. You can move out but you would be liable for the lease if the roommate doesn't pay or comes up short. But that may be a small price to pay for your safety.