He is certainly a hero. Danzō Shimura is the the phrase "The end justifies the means" incarnated. Yes, he participated in some morally questionable actions. These included the 'Assassination of Yahiko' and the 'Uchiha Clan Incident' and his actions during the 'Invasion of Pain' were debatable but all of them had a greater reasoning to them. Look at the Akatsuki at the time - Danzō & Hanzō's Goal was to eliminate the Akatsuki Threat - they weren't such a threat then, but could easily become one later on. Had there plan succeeded, Nagato would have died and Akatsuki would not have become as big as its potential allowed it to.. a major aspect of the Mugen Tsukuyomi Project would be shut down as well. In addition, the Uchiha Clan were going to press harder and harder before the invasion - extermination at that point was the only option and Danzō was completely justified to keep Naruto away from the Hidden Leaf.
Letting Tsunade die was a bit harsh but I think it is a morally justifiable in a sense - Tsunade was a very soft-leader. She may not appear that way, but she is - she wanted to bring back International Enemy #2 [Sasuke] to the Hidden Leaf, a village he sought to destroy, believed in Naruto and wanted to put Naruto and Kirabi into the war. Hiruzen was an earlier example of a soft-leader. His policies led to the Hyuga Clan Incident, Invasion of Orochimaru, Two Shinobi World Wars which led to the Kyubi Incident which led to the Uchiha Clan Massacre. Danzō doesn't take risks - he was not a extremist. He was a Utilitarian Enthusiast who followed more Right-Wing/Conservative perspective..
