Razman pretty much nailed it in the first two sentences.
It's to gain power:
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Collaboration with Orochimaru (who wanted to destroy the village at the time) and the secret experiments that was going on between the two was to set Danzo up for being hokage.
Danzo knew about Orochimaru's plan and he waited out until the hokage got killed.
Kind of like what he pulled during the pain envasion.
I agree about Danzou wanting power himself but his collaboration with Oro was mostly opportunistic with both doing work only if it served their own purpose too. And I will come to that later but what would Oro get if Danzou gained power? I doubt Danzou would let a rival like Oro back in village and Oro was definitely not naive enough to trust Danzou.
Danzo was never allied with the methods of Hiruzen, the coup was more advantageous for Danzo [In order to gain power - working with Orochimaru]
If Kotoamatsukami prevailed, the coup would've been cancelled for even a decade; thus prolonging Orochimarus plan in order to invade Konoha [Itachi would still be working under Root] and in meantime another good candidate would've been selected as the Hokage
The coup d'etat had to be continued in order for both of em to achieve what they wanted successfully [Danzo & Orochimaru]
It was all undergoing effectively until Hiruzen took away Ninjutsu of Orochimaru [Plan to get the vessel failed], thus prolonging Danzos idea to improve his arsenal [Hashiramas DNA and Izanagi]
P.S. Include the fact Danzo wanted to threaten Itachi using Sasuke as a medium [He also wanted his safety], since Orochimaru was aware of the potential main lineage of Uchiha clan carries .
They had a connection from the old days when Oro was still in village as Oro took part in covert operations and Danzou oversaw those.
But seeing Danzou always wanted the hokage seat for himself - it might as well be him who gave the tip about Oro's activities to Hiruzen. Of course it's just a probability and guess work, since manga ever dropped any hint regarding it. ( If it did I don't remember. Still need to read it back). In absence of any detail in manga it's hard to say how much Oro was invested in Danzou being a hokage after he had to leave.
As for Danzou taking Shisui out of the equation- that act was clearly to undercut Hiruzen's attempt to reconciliate with Uchihas. For, if Hiruzen managed to settle the issues with them then it would not only Uchiha would benefit- Hiruzen also would have made his own position stronger. For :
1- He would have successfully avoided a civil war without any violence.
2- Would have gained Uchiha clan support in the process.
3- Uchiha clan's removal weakened Konoha considerably making the hokages constantly stretched thin and worried over how to keep the facade of a strong Konoha despite lack of enough nins. I doubt with Uchiha clan alive Kumo would have tried to kidnap Hinata from the village and then demand Hiashi. Hiruzen wouldn't compromise on it if he had enough power to turn the threat back. So Uchiha clan removal was not in Konoha's interest.
And later on we also see Tsunade constantly trying to manage the missions despite not having enough man power. It was said that turning down the mission not only meant financial loss but it would also be accepting that Konoha didn't have enough men to handle it, thus highlighting the dire state of the village. So it's hard to believe Danzou did all this for interest of Konoha alone.
Danzou had already challenged Hiruzen for hokage seat and lost. Then he had teamed up with Hanzo to wipe out akatsuki lead by Yahiko in exchange for his help in Danzou becoming hokage. That one again backfired when Nagato used his rinnegan and wiped out most of his alliance with Hanzo. After that he tried to get Sasuke killed..
I am not sure what he was thinking - was he really thinking about Konoha or to save his own skin? For there was always a chance that Sasuke might find out the truth about how Uchiha ended up dead.
And then he tried to weaken Tsunade by killing of the messenger frog instead of helping out in fighting Pain. With his sharingans he might have been an asset but of course it would also reveal what he had been up to so it wouldn't have worked for him.
It was not until near his death, he truly decided to act like an honest nin and sacrificed himself instead of giving Konoha's enemy another weapon in form of Shisui's eye. He put himself before Konoha despite all his special brand of nationalism before that. At that point he realized why Hiruzen was always a step ahead from him in his leadership despite him believing himself at par otherwise.
And no he was nothing like Tobirama. It was more like Hitler using Nietzsche philosophy for his own agenda and make Nietzsche look like a Nazi when the poor guy wasn't.
I agree with almost all you wrote.
I mean there was another reason why danzo wanted to get rid of Uchiha was that noone will interfer when he try to take control over Kyuubi but it would be long time set plan. For short time plan it only to make sure he became Hokage and gain as much power as possible.
and I disagree with some parts of Orchimaru && Danzo coopearation as Orchimaru broke contact with Danzo after leaving Konoha sure. It was Danzo who tried to find Orochimaru and approached him and bothered him, no opposite way.
It only looked like that due to both of them having some common interest/wishes. "Enemy of my enemy is my friend"
Seriously they tried to use each other for own benefit then get rid of themselves when opporunity would come.
Sai mission to assisnate Sasuke was for sole purpouse to not allow Orochimaru gain more power then Danzo.
Orochimaru giving Danzo's boost(Senjuu arm and sharingan) was long time test for Orochimaru to do deep research and use it in future, it was also way to make sure that Danzo wouldn't betray him and reveal Orochimaru secrets in Konoha.
^That looks more like it.
