I believe Izanami can kill anyone at anytime by simply creating a loop in their head and then cutting their head off. Itachi states that he used it since the first attack.
Now if this is true he could have been used it at any moment. Now the reason he didn't is because he wanted to teach Kabuto a lesson and stop Edo.
So this means Izanami is ridiculously overpowered and Itachi can just create any type of loop and kill anyone.