Leave me alone and eat your stupid Scotch egg.
It's not really a theory but a really good plot twist was that they find and kill the last -A. Before they die they tell the girls of someone who has been watching them since the very first text. This is the worst -A because there are no threats but you always know they are watching you. The girls eventually find them. They shoot them and the screen goes blank. We, the audience, are -A. We have always been watching them even though we didn't know ourselves. Sorry if it's a bit rubbish xx