That because courtney took alison's place and ruined her life while she was in a mental hospital. So she killed courtney who looked just like her sister alison and found out that her sister put the four friends secrets into a diary for her also to become the real A.
Courtney is alison's twin sister, who pretended to be her
Suffication
by hands
Alison
Yes
Courtney wants to pretend to be Alison meanwhile Courtney was supposed to be in a mental hospital
Yes. Alison had a twin sister named Courtney.
she tried to be alison and she was in a mental ward
yes
Yes
Courtney stole Ali's ring :) hope it helps
Because the Pretty Little Liars weren't actually her friends. Courtney, (Alison's twin sister) made friends with them. Alison had nothing to do with them. So she's mad and tries to kill them.