Who Is the Mysterious Stranger Killing All of the Raiders?

So, recently, I decided to play through Fallout 4 again and was doing a rescue mission for Preston where some raiders had kidnapped a girl from Abernathy Farm. They were holed up in some old building. When I got there and entered into the building - to my surprise - a gun fight was already taking place. I hear the raiders yelling and moaning, basically getting massacred, and a Zapp Brannigan sounding guy saying things like, "Sorry, you should have walked away." and "That was easy." et cetera, but I could never get a clear look at him in the shadows and would only slightly catch him from behind as he was walking into another room leaving a trail of dead raiders. Honestly, I was kinda creeped out! XD

I tried to stealthily follow him hoping I wouldn't have to fight him and ended up walking into one of the raider's grenade booby traps. When the game reloaded I immediately hurried back to the quest but this time he was nowhere to be found :(

Has anyone else experienced this encounter while playing or could explain?