Verified answer

In the end of the movie, the woman despite being stabbed multiple times wasn't dead, only her husband was.

When the 'strangers' said "It will be easier next time" they were talking about the next time they terrorize a household, which leads into the sequel they are currently making that starts off with them at another house.

She wasn't dead yet. She died after that last scream.

They took the pamphlet and said "It'll be easier next time" because the girl was having a hard time with what they did. It looked like her introduction to the family "business." She was having a conscience attack, and they were trying to talk her through it.

Leaves a lot of room for a sequel.