The most confusing thing about Shana…

Okay so Shana felt so strongly about Ali that she left her home, her family, her school and friends to move to rosewood to help Alison. Then falls so in love with Jenna that she wants to kill Alison for her.

But the most confusing thing for me is Shana could’ve killed Alison long before the liars knew she was alive. It would’ve been pretty easy to get away with a murder that everyone thought was committed years before she came to town. She even helps Emily meet Alison. So why wait until not only the liars know she’s alive, but when they are with her too??? Unless her plan was always to kill them all?