Uday bro, If u are in lease and he is not then u have rights for anything. He cannot sue you at all. Like Madhesi mentioned you can ask your apartment manager to change the lock, if you don't wanna pay for the lock then you can tell your apartment manager that your car got broken into and stole your apartment keys, there is 99% chances they will change the lock for free. And also while talking with your Manager tell her/him that nobody has permission to enter in the apartment, this will prvent your roommate to get the keys from manager. Just in case your roommate is in lease, go and talk to your apt manager again and tell them that from now you will be paying your part only, this will led your apt manager to send him letter about his part or ask him to terminate the lease. There is nothing to be scared- if your roommate is not in a lease then you got the jackpot-you can even call cops and they will take your roommate away or get the apt keys from him/her.