Board Thread:Naruto Discussions/@comment-26636534-20180325094649/@comment-5085906-20180325141512

The explanation is simple. You're confusing the manga with the novels.

Manga: Created by Masashi Kishimoto. Began in 1999. Ended in 2014. Is canon.

Itachi Shinden novels: Original stories written by Takashi Yano. Released in 2015. Not canon.

If you're so sure Itachi killed Izumi in the manga, then surely there must be some panels you can show me.