This guy is characterized as THE embodiment of corruption in the leaf

Which I think while is a necessary character design, I don't think it's a realistic one. Aside from him, there's very little corruption beyond ninja that have already been banished like Orochimaru - like other than Danzo and by extension Root (who aren't corrupt by nature like Danzo since they believe they're serving the Leaf by serving Danzo and are pretty much all brainwashed into unwavering loyalty) - like you're telling me no one else in the Konoha aristocracy or legal system was a power hungry, or at the very least, sleazy bribe-taking figure?

That said, it's shown and characterized so many times that the only thing he really gives a shit about is becoming hokage or having the power of one. Before the Uchiha massacre, he steals Shisui's Kotoamatsukami Sharingan. He's shown using it on the 5 Kage Summit. Bonus points, one of his bodyguards is Fu Yamanaka who has body possession secret technique. The weakness of that technique is shown in that it doesn't really give you the memories of the victim to show how they act. Sharingan has been shown numerous times to have some manner of mind reading ability, as evidenced by the fact that Obito used it to find out Nagato's burial grounds from Konan.

Koto Amatsukami is described as being much more subtle and more powerful, by using genjutsu to give the victim false memories/experiences and presumably act toward's the caster's will in a fashion within their personality. The technique is very sparingly used in the series so there's not too much to go off of - we see in the 5 Kage Summit, Danzo uses it but Mei starts getting suspicious of him and Mifune. That's all well and good but consider it being used in the Konoha - where no one's really going to think twice since he's the most (legally) powerful person in the village as opposed to the 5 Kage summit where you have a collection of powerful people from all villages.

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So... Why not? For a normal user, using MS techniques often makes the user go blind, but we're shown that isn't an issue for Danzo, who's souped up with the series' favorite plot armor drug in Hashirama Cells. It's suggested he can't use it several times per day, but why would he need to if once per day is enough to set massive plans in motion - evidently Hiruzen believed one use of Koto Amatsukami was enough to stop the entire Uchiha revolt in it's tracks, or at least delay it for a long time.

(This isn't even looking at the fact that he has Fu Yamanaka as a subordinate who he *might* be able to use as an "oh shit" in clutch moments. (I mean it isn't likely but it's a non-zero possibility since it runs the risk of making them incredibly suspicious as to why Danzo had them put in a bodyswap technique)

Any responses like "Hiruzen was his friend, he wouldn't want to do that to him" don't suffice because he literally tried to assassinate Hiruzen in the whole Kakashi/Itachi Anbu arc.