At the beginning of the film, we see Kristen and James' relationship ultimately suffer. They don't know if they are meant for each other or not and would have parted ways..that is until three assailants invaded their home..
Since then, both Kristen and James have been desperately doing everything they could to survive their own home invasion. Ultimately, they run out of ideas and are slowly being backed into a corner as the film progresses. The film hits a chilling climax when Dollface whispers "your gonna die Kristen!"
When they are finally taped to a chair and ask them "why are you doing this?" They respond "Because you were home"
What's my interpretation of the ending? Kristen and James finally accept their fate as they hold each others hands in the process because after everything they've been through, their love for each other was strong enough to take them into the afterlife. There was no longer any room for doubt or denial. Their security wasn't in their home as is believed by many Americans, but in themselves.
As for who the three intruders are, we will never understand completely but the ending gives us a clue. When the two boys passing out religious pamphlets, Dollface gets out of the car to collect one. When the boy asks her "you a sinner?" She replies with "sometimes..." Her partner in crime then says "it will be easier next time.." Despite everything that happened, i believe that Dollface was deep down a guilty human being pressured by her other two partners to engage in a life of crime. It is in this one scene where we don't see her identity, but we see a glimpse of the kind of people the killers are.
Overall, the Strangers is one of the rare slasher films with such mystery and revelation culminating at the ending.