You are being too judgemental here. A dead date might not offer much in the way of conversation, but they make good listeners.
I had this happen a couple of times and I agree with previous posters that you should contact the authorities. You might also want to let anyone that was around that might be some sort of an authority figure know about the situation as they might have also had other people with similar stories about this person. You mentioned the library so you might want to give them a call and let them know about this person and give them a description. You would be surprised how people will change their behavior when the police have a little word with them.