Mona's motive is plain stupid if I got it right...she was jealous of Hanna that got back with 3 other girls but those A messages brought them back together
Idk we still haven't found out who killed Ian and Garrett so ....
@popboi__ Marlene herself said Mona is and always will be original A. Once this new person stole the game from here in season 3 she wanted to expose him/her but didn't succeed