It has to do with eating the Yami yami no mi fruit, which is the strongest fruit and has the ability to absorb the energy of other fruits, i.e. to subdue them
It's true that Danzo always had the ambition to become the Hokage, but on the other hand he was a friend of Hiruzen. That's why he would not try to dethrone him when the 4th died
Itachi is a tragic figure. In my opinion, he should have clashed both with Danzo's murderous plans and with Uchiha's coup de etat attempt, bring it all in the open and let the village decide democratically what to do