The Identity of "A" Was Finally Revealed in the "PLL: Original Sin" Season Finale

There was never any doubt the Liars' mothers, who, at one point, were friendly with Angela, had something to do with her tragic suicide at the NYE party.

It seemed possible one of the five friends resented the rest of the group for being awful to Angela and decided to exact revenge on them as a way to assuage the guilt she harbored.

Since Davie dies in episode one, and every passing episode reveals the specific ways each mother relentlessly bullied Angela in the past, the only mother whose history with Angela lent itself to this theory was Sidney (her only crime against Angela was doing nothing to help her). But in the end, this simply wasn't the case.