That's where you're wrong!! So wrong!!
What did Danzo sacrifice? His life that was already over? He was seconds away from dying, that's when he pulled out the seal!
Danzo was stabbed by Sasuke once, and he took Karin hostage to survive, that's when you find out about his true ideology!
Sasuke asks him: "Isn't it strange for you, the one who preached about self sacrifice to take a hostage?
Danzo answers: "I am the only true saviour of this world, and I am the only one who can reform the shinobi world, for that cause this woman will be sacrificed".
Do you see what he really was now? Do you even know how many members of Root were sacrificed for that very reason?
Then Danzo is stabbed through Karin in his lungs by Sasuke's Chidori.
He was going to die in a matter of minutes! Danzo realised he had no more chances left to live, and so he activated his seal!
This is another quote by Kishimoto: A shinobi is not measured by how he lives, but how he dies. It is not measured by what he accomplished in his lifetime, but what he accomplished with his death!
Now tell me, what did Danzo accomplish with his death? Absolutely nothing!
You should learn to perceive the situation, not just read the words being said.
BTW Danzo didn't choose to fight Sasuke.....Tobi forced him to!