From your conversation above, it seems like you signed a lease with your first roommate, Roommate A. It sounds like your current roommate, Roommate B, is a subletter of Roommate A.

You and Roommate B would be on the same lease in this case. Under common law, the landlord can go after you, Roommate A, or Roommate B for rent in its entirety, or damages to the apartment. It's possible that the lease stipulated otherwise, but, honestly, that's pretty doubtful. When you say that when one person doesn't pay, the other isn't held responsible, I assume you mean the landlord contacts the indebted party about payment. That doesn't mean everyone else is off the hook. It just means the landlord is a rational human being that doesn't sue all of his tenants immediately upon lack of payment.

That being said, I'm not sure if you're worried about current damages. It doesn't sound like anything was damaged.

What are you asking?

Are you asking whether you can ask your landlord to evict your current roommate? Sure, you can ask. Might not be a great idea, for any number of reasons.

Are you asking whether you yourself can evict your roommate? Probably not. Some states allow you to seek remedies, such as orders for protection, in cases of domestic abuse, which would essentially result in your roommate's eviction. That being said, I don't know where you live. The conduct you've described doesn't seem to rise to domestic abuse in my state (MN).

Are you asking if you can get out of your lease? Again, you could ask your landlord "pretty please," but it's unlikely that he'd let you go no strings attached... especially if he's aware that the remaining tenant is a bit incompetent. Your lease may have terms allowing for early termination of your lease. In fact, your state may require that such terms exist in your lease. Those terms may be statutorily dictated. Often times in MN, a landlord will ask for two months' rent as a penalty to leave a residential lease early.

Did that help?