An unfortunate event involving a fellow New Englander and likely New England hiker.
A trailhead shooting would lead me to believe they were followed from maybe a breakfast spot or maybe getting gas. They called it a popular trailhead but the next people to wander by did so three hours later, sounds pretty remote. (What's the time you'd wait at PNVC, Franconia Notch or Appalachia? How many would hear a shot outside of 2:00 AM and during the summer, you may have a Traverse starting and someone close by)
If his gun is out, I can't see him waiting for you to draw yours, pull it from your pack etc.
While following someone apparently hiking will lead you to a possible secluded area (a good place to commit a crime) it's possible they pull into a PNVC or Crawford Notch with lots of people milling about. Maybe the criminal overhead them and knew the area would likely be empty?