When I say show/series I mean the show as well as the books. It's obvious that PLL is a story about revenge. People who have an axe to grind with the girls are ganging up on them. But what's the point of doing it in secrecy? Why not get them all together in some abandoned warehouse (since they can get the girls to go anywhere), reveal yourself to them and then finish them off. Why go through the trouble of messing with their lives and staying hidden?
Btw I have not read the books so this isn't a spoiler. This is merely a review based on what's been revealed on the show already.