Danzo was the biggest problem!
Danzo may have done everything he did for the leafs sake but most of the time he took it way to far and suprisingly enough, is responsible for alot of problems and the rise of many major villians. A few examples are:
Sasuke-It was Danzo's idea for the Uchiha Massacre to happen, and ultimately this turned Sasuke into a major villian and led to his own death.
Nagato-Danzo worked with Hanzo to eliminate the earlier Akatsuki and caused Yahiko to die. Yahiko's death caused Nagato to form the newer Akatsuki and eventually destroy the Hidden Leaf Village.
Kabuto-Danzo threatened to shut down the orphanage that Kabuto lived at if Nano didnt complete a mission for him. This also led Kabuto to become a spy and meet up with Orochimaru, and become his right hand man. Eventually becoming a major villian during the 4th Shinobi War.
In conclussion, Danzo is a cruel person who (with or without knowing it) has caused major villians to rise all by himself