1.) Hiruzen was against it and he did it against his orders. You really can't expect peace from violence, I think killing them was the incorrect thing to do. Being able / willing to negotiate is important if you want to be the head of a village.
2.) This proves that he is a control freak and will do what ever it means to be in charge, he used Hanzou from the Amegakure and even killed the frog Kousuke, the one that Tsunade sent to inform Naruto of Pein's attack on Konoha so that the village would be in a weakend state. A lot of people in Konoha died from a result of his decision and would have remained dead if it wasn't for Naruto "changing" Nagato.
All of this proves that Danzo is a man who doesn't intend to keep his promises, and that he is not worried of threats that would hurt Konoha. He is a cheap, dirty man who needs to exiled from Konoha / eliminated for the village's safety in my opinion.