Omg! Mona is “A”, it was a shock to me I had to admit because she was on their side, but then I thought the saying “keep your enemies closer. But I also think it was a group of people comprised of Spencer’s sister and others who hated Alison. I assumed “A” was Spencer’s sister because she was shady from day one, but she might in a group of people comprised of Mona and others called “A”. There’s no way one person could have done all that. Anyways, they kidnapped and killed Mya to get to the liars again. The liars think it’s over but it’s not, like Mona said in the voiceover.
She wanted to recruit Spencer into the “A Team” and turn her against the liars, but Spencer refused. Mona went nuts because she used to be a geek and Alison tortured her, so she started plotting against her and joined “Team A”; Mya became their next target, that’s why Mona’s not upset about getting caught.