What are highway men? What kind of people would resort to such behavior in this type of disaster?

From the text:

It was strange that the term highwaymen had revived in its true and literal sense. These were not the romantic and reputedly chivalrous highwaymen of Britain's post roads in the sixteenth and seventeenth centuries. These new highwaymen were ruthless and evil men who lately had been choking the thin trickle of communications and trade between towns and villages.

Highwaymen are generally in it for themselves. They see an opportunity to control others and possibly make money. These men are ruthless and uncaring.