Why was Ali killed in Pretty Little Liars?

Answer 1

This is rather confusing...

Okay. So the DiLaurentis's actually had three children; Jason, Ali, and a twin sister of Ali's, named Courtney. While Ali lived as the popular girl, there was a school activity called "The Time Capsule". There are many pieces of the flag hidden, and Ali claimed to have one, because her brother, Jason, told her where it was.

The PLLs (Spencer, Aria, Emily, Hanna) were nobodies, and Ali had befriended the most popular girls in school, Naomi and Riley. All four of the girls had showed up that night to steal Ali's flag piece. Courtney (pretending to be Ali) had said that they were too late, as the flag piece had already been stolen (apparently by Jason, as Aria later reveals). Ali and Courtney had switched places, and the Dilaurentis parents didn't know.

Later on, Ali is sent to a mental hospital (the very same one that Hanna had been sent to by her father) and Courtney takes her place in school. Not knowing who Naomi and Riley were, she ran to befriend the 4 PLLs. Ali later gets revenge on Courtney for ruining her school life and pushes her into a deep ditch to kill her, during a sleepover in Spencer's sister's barn.

(side note - Mona probably didn't actually have any interaction with the Ali that killed Courtney.)

In other words, Ali was NOT killed in the ditch.

BUT.

The DiLaurentis parents announced that they indeed had another daughter... one that no one (including the PLLs) knew about. Her name was Courtney (who was actually Ali). She tells all of the PLLs and brings them to their beach house as a "celebration sleepover" for reuniting. While they were sleeping, Ali traps them inside the house, to try to kill them. She gives them a note that says that they ruined her life. This is when the PPLs smell smoke and try to escape the house, using a secret passage that they used to use for scaring Jason DiLaurentis, where they see Ian Thomas's dead body, and Spencer's older sister, Melissa (still alive). Barely escaping, they watch the house blow up, with the REAL Ali still inside.

Sara Shepard leaves the reader hanging in the epilogue, when she says that a new student has moved into your school, as an anagram on Alison DiLaurentis, suggesting that she is still alive.

Long story short...

Ali supposedly didn't die.