We know that Lucas was visiting radley when Mona was still in her drug daze and we all assumed he was visiting her but what if he wasn't. What if he was visiting Charlotte, he could have met and fall in love with her. I mean think about it he visited up until Mona started to get better and play the game again which if the timeline is right was the same point Charlotte escaped. Which is why he didn't need to visit anymore because she was out. It also explains why Toby tried to run him over and who the A was that Charlotte was giving key's too etc.
Present day think about it, mostly everything A.D. uses against them is talked about in his apartment(he has it bugged) and this A is small and needed to drag Jenna like he would. This A also left floors at her grave and have a black town car and we know he has money. Plus this A wants revenge and what better motive then having your love taken away.