Who killed Ali in Pretty Little Liars?

Answer 1

In the 7th book, Heartless, it is suggested that Billy Ford killed Ali. In the 8th book of the series, Wanted, the truth is revealed (in somewhat retroactive continuity).

Ali secretly had a twin sister, Courtney, who the girls knew as "Ali" until she was killed, actually by her sister the real Ali. Courtney was pretending to be Ali while Ali was in a mental hospital overseas. When Ali came back, she was so mad at Courtney for pretending to be her that she pushed Courtney into a hole, killing her. Aria, Emily, Spencer and Hanna didn't know about Ali's twin, so they were led to believe that the real Ali died.

Mona was originally "A" because she was jealous that Hanna dumped her for Aria, Emily, and Spencer. Mona was "A" for awhile until she was killed at Homecoming. After Mona died, Ali took on the "A" persona.
It wasnt Ali that died. It was Courtney.
Well, in the books, Ali had a twin sister Courtney who switched places with Ali, then Courtney became friends with the 4 girls. Ali was mad at Courtney because of the switch so ali killed Courtney that night. and the REAL Ali was alive and courteny was killed.
First of all Ali was really Chelsea, Allisons Twin sister. They switched places because Chelsea wanted a normal life ( was in a crazy house). So to make the long story short, Ali Killed "Ali"

Read it in Wanted, The Last Book