The Dark Half
I was in a remote cabin in a heavily wooded area with a dirt road going to it with a guy with shoulder length blonde hair and two girls, one brunette, the other blonde. The two girls were there under some kind of witness protection programme after an attempt had been made on their lives but they weren’t aware of anybody who wanted to harm them or any details of the attacker.
The guy in the cabin turned out to be a sleeper agent who on some commands embedded into a song that sounded like intentional lyrics as part of the song, but with sinister overtones would activate.
He was aware of what he was doing but powerless to stop it. He produced a knife and while comforting one girl proceeded to loom over the other. His reflection in a mirror of the gleeful attacker while in reality he looked on in terror while the first girl was unaware.