Danzo. Sacrifice.
Danzo remembers clearly the number of deaths he's authorized over the years. The exact figure is spectacular, hundreds upon hundreds, remembered in blood and bone, ash and hellfire, poison and ice; and he suffered each one with a blunt harshness and absolutely no mercy. He is called many names for it – monster, evil, tyrant, Uchiha Shisui had been particularly creative – and he accepts every single barb and all the furious, hate-heated gazes with contemptuous silence. Because it is Konoha at stake.
The reality is that, in the world of shinobi, every peace is unstable, and complacence is a death warrant, and he'd gladly eat his own heart if given absolute proof that foreign underground forces weren't mobilized at every moment, usurping each other. Sarutobi could blather on about trusted alliances and his student could go about prophesising world peace, but it is Danzo called the pre-emptive strikes which turn the tide of war in their favor, it is he who did all commanded all the torture and espionage so they might have information, and it is he who taught his men suicide tactics that they might protect their own.
It is Danzo who weighed faces against numbers and friends against a mass of faceless strangers and chose the latter.
(The thing about martyrs is, they always die hated.)