Yes, it is illegal to leave a hotel without paying the bill . A person who does that can be charged for theft of services . This gives the hotel authority to take action against you. They can even call the Police and you can be fined and even punished for the theft of services.  Hotel authorities take full details of the guest when the guest  check in into a hotel . So if someone tries to escape without paying the bill they can easily catch him/her  or can also file a complaint against the person. Make sure that all your dues are clear before leaving the hotel to avoid any kind of inconvenience or difficulty .