Danzo was one of the best and more truly twisted Naruto Villians, with his schemes reaching far enough to have ties with horrible people like Hanzo, being able to twist things from other villages and lands. In some ways, I’m sure you could link him to almost all the notable horrible things happening in Naruto.
He helped twist Orochimaru, encouraging the darkness growing in him, he murdered Kagami, most likely had ties with Sakumo failing his mission since he was a ‘threat’, most likely leaked information to enemy villages to distract people from his plan for power.
He was poison to more than Just the leaf village, his touch having a waving effect on most of the ninja world in some way.
Minato threatened his position and activities, and when the nine-tails attacked his plans grew in deeper, because now he had a trap set up to make Hiruzen powerless against him. His checkmate insight, knowing that only a few issues remained, those being on living Hatake and the Uchiha clan, that his secret Anbu members were watching to cause more distrust in them and make them poke the hornets' nest.
He killed an entire Clan for power and put Hiruzen into a stalemate, so he couldn’t do shit about his underhanded bullshit.
bonus: Hiruzen wanted to deal with the root problem, but in his old age and the trap Danzo sprung he wasn’t able to do much, but he really did try and feels guilty he couldn’t do more without risking even more slaughter since Danzo obviously doesn’t have any issue for innocence people dying for his cause.