Danzo is one of the most hated characters in Naruto. The reason being, he stole Shisui’s eyes, was responsible for the death of Yahiko which led to the formation of the dark Akatsuki and many others.
So why did Danzo take Shisui’s sharingan? The answer is pretty simple. The Uchiha clan was unhappy with the village elder’s descion to push them to the border of the village, hence they planned coup d’tat against the Hidden Leaf village.
Uchiha Itachi and Uchiha Shisui were the only ones who were against the coup d’tat.Shisui was planning to stop the coup d’tat with his Koto Amatsukami, but Danzo didn’t trust Shisui and hence he stole Shisui’s eye.