Arsenic has a maximum recommended concentration of 10 parts per billion, according to the WHO. In order to make a sword of the stuff, you'd need to alloy it with something else; lead comes to mind. The concentration of a lead-arsenic sword would probably be over 10 PPB, and would not be a very nice thing to get stabbed with. Come to think of it, stabbing people in general isn't a very nice (or safe) thing to do....