Board Thread:Theories and Speculation/@comment-34656247-20191119070420/@comment-37982756-20191124142832

They slaughtered so many Uchiha's and none were mentioned to resort to Izanagi, Sasuke didn't resort to it when he got stabbed by Madara. Maybe Itachi thought that is what would happen. Except Obito freely could, with his sharingan stock, which is something he didn't know about.

Izanagi is a forbidden technique that very few shinobi even among the uchiha possessed any knowledge about. According to Itachi himself, Izanami and Izanagi form a pair. So it's fairly obvious that he possesses knowledge about both the techniques. Itachi thought the masked man was Madara. How would he setup a fail-safe without knowing what his foe's abilities were?? Itachi either didn't know the full scope of Izanagi i.e, time delayed activation (knowing Itachi this is extremely unlikely) or he set a kill switch in his brother's eyes to counter Izanagi but failed anyway because he had no clue about Kamui. The latter seems like a more reasonable explanation to me.