There's no mentioning in the seventh book that they began to use any of the Unforgivables, just that a curse killed her. There are multiple curses that can cause death other than the Killing Curse. User:Simen Johannes Fagerli.
- Actually, Aberforth directly says that Grindelwald cast a Cruciatus Curse, so Unforgivables were cast. However, I believe you are right, I don't think it was ever said Ariana was killed by Avada Kedavra. -- 17:08, December 6, 2013 (UTC)