Board Thread:Theories and Speculation/@comment-30172350-20170617091148/@comment-734582-20170617150335

Ririblob wrote: the worst things orochimaru did danzo didnt know about he just wanted strong (and unethical but that wasnt the point) jutsu to protect the village he used orochimaru as long as he needed him and whats wrong with izanagi its just a powerfull jutsu Danzo knew Orochimaru. He may not have know details, but Danzo was well aware of the type of experiments Orochimaru did. If Hiruzen had known the number 1 enemy of Konoha was secretly being supported by Danzo, Danzo would have been executed on the spot.

Sure, Hiruzen likely suspected Danzo may do more fishy things than he let on to. But actual treason wasn't one of them until Danzo revealed his true colors with the Uchiha Massacre.