How deadly are the nuclear submarine's missiles?

Their yields are about the same as most land based ICBM warheads or bombs dropped by airplanes: from 100 kiloton to 300 kilotons. Yields in the megaton range as were common in the 1950s through 1970s have been shown to have few real military uses and more damage can be done using several bombs of less total yield at lower cost using warheads in the 100 kiloton to 300 kiloton range.

Deaths will depend on too many variables outside the characteristics of the warheads/bombs themselves, including the target, burst height, availability of shelters, emergency preparedness of the population, etc.