How can Mona be A?

Answer:
Because she read Alison's old diary and discovered all of the Pretty Little Liars's secrets and information. Plus, she's evil and she wanted to haunt and scare the PLL.
First answer by Xoxojenxoxo. Last edit by Xoxojenxoxo. Contributor trust: 1 [recommend contributor recommended]. Question popularity: 1 [recommend question].