I think they might have worked in pairs, especially going into FR's place, a home with 4 people (2 children) in it.
Wouldn't one person wake, jump, scream, try to run - if the person sleeping next to them was shot?
And what if in any house someone got up to use the bathroom, or simply couldn't sleep? And unless light was streaming in through a window (was it a full moon that night?), how did the killers see clearly - they wouldn't risk a light switch.... were they wearing night vision gear?
How scary is it to be related to this family or have had business dealings with them, knowing now that they're capable of this.....