While I agree with both of your assessments, in part (not only armed populace, but also Spectre gunships and Daisy Cutters would wreck havoc on any swarming horde), keep in mind that over 6000 people die a day on average in the U.S. Immediate containment would be imperative, but not outside the realm of possibility in major cities for an outbreak to get out of control, at least initially.
The most realistic scenario is the one put forth in “28 Days Later”. A virus that renders people violently insane in under 1 minute and is spread by bites & bodily fluid. It would be very hard, if not impossible, to contain such a thing even with weapons of mass destruction.
The dead coming back to life (let alone being able to run) is just pure fantasy. But wouldn’t put it past some nation state to develop a “Rage”-like virus.