Why did Isla want to kill jessica and david?

First, you should edit the spoilers out of your title. Second, the best I got was she loved all the devil crap her father had so she became a satanist.

It's explained in the film that Isla promised the Demon 3 souls. Each person she cursed was supposed to kill someone, then commit suicide. David, while he did stab someone, never got the chance to kill anyone or himself, because Arne invited the Demon to possess him instead of David. Arne kills Bruno, soul number one. Arne then tries to commit suicide. Jessica kills Katie in the woods, soul number two, and then commits suicide by jumping off the cliff. When Ed breaks the altar with the sledge hammer, he broke the curse. Because Isla could no longer complete her task, the Demon appears and takes her soul as number 3.