My new theory is that it could be a raider or secret police sent by some other Silo/actual people in charge? We know that all IT Silo factions have external power and some sort of ability to rely info (the key chain and the fact that the Legacy is monitoring most all Silo activity in real-time. So they saw whatever Bernard saw thru the visor as she made her way over the hill, thru the bodies and into 17. They know they'll lose 18 if she figures out how to go back, so they sent an assassin of sorts to stop her. It is prob far more likely it's simply a resistance member that lived up in Judicial, but thought it would be a cool twist?