Why do the killers reveal their faces to the victims? It's a matter of taunting. Before that, the couple might have had some hope. Maybe these three are just creeps or sadistic burglars who are just going to leave after they've done what they want; after all, James and Kristen weren't killed outright even though they were absolutely helpless. But revealing the trio's faces means they have no way back since the victims may now describe them to the police. It says "folks, you're gonna die now" louder than the knives in their hands.
So get this: apparently, in the Blu-Ray commentary, the reason the Masked Man looked at Dollface when she said the infamous “Because you were home” line wasn’t out of confusion, what I initially thought was him checking to see if she was getting cold feet, or even him doing the Michael Myers head tilt. No, evidently, it was the other way around: Dollface was getting off on it, and MM and Pin-Up were less willing to go through with it. Let that sink in: Dollface, the youngest member of this little trio of psychopaths, is the cruelest of all of them. According to the person who told me this, it gets “better”: Kristen and James were apparently their first kills (though Chapter One will likely call this into question).
