Theme

A central theme of Bad Girls Don't Die, by Katie Alender, is revenge is bad.

Sarah is angry and wants to kill the people connected to her past.

Since Sarah is angry, her anger gets ahead of her. She thinks killing the people connected to her past and making them listen to her will make her life easier and slightly happier. However, her revenge didn't go as planned. Her spirit burned in the house. Since she has already murdered someone, her bad behavior has led to her punishment.

Her spirit burns in the house.

If Sarah had chosen forgiveness, changed her bad spirit, and accepted her fate, she would have kept her spirit and made it good.

At the end of the story,

Shara doesn't choose revenge and helps others.

Shara was blamed and was never wrong. She was never a bad person. Shara never wanted revenge. She helped the people who Sarah tried to kill. Her good spirit was strong and defeated Sarah from getting to Kasey. When the house burned, the spirit in the heart necklace survived. 

Her spirit in the necklace survived and made it out the burning house.

If Shara had chosen revenge and was bad like Sarah, her spirit would've probably burned in the house too.

A central theme of Bad Girls Don't Die, by Katie Alender, is revenge is bad.