At this point, it seems to me like the story of A is somewhere between the Toby's mom suicide/Radley storyline mixed with the Hastings and the DiLaurentis families. There's a huge piece of the puzzle there and I don't think we'll be able to figure anything out without it.
But one thing for sure, Big A exists because of a very dirty secret/matter/etc... between those families.
If you think about it, A has only really gone apeshit after Mona went to Radley and 'lost the game'. And more even, with Ali's return in Season 4.
During the first season, A was only sending text messages and disorienting the liars, a la Gossip Girl (a dark one but still). He really wasn't that dangerous, vicious, and murderous. The whole Ian thing was mostly the liars jumping to conclusions and believing them so hard.
So, either A has been the same person all along, amping up his game and spiraling downward towards obsession/insanity; or, Mona started it, Shana went at it for a while, and then 'the game' fell into someone else's hands, someone really dangerous and with a lot of motive.
Was it Mona during the first season? or did she join A in the second season? Did she originally start it when she helped Alison to fake her death and had that doll lair in the motel room? I've lost track of all these details