The answer is God is in charge not man and if he deems that war is necessary then we as a nation must complete it.
Lincoln's speech was not about gloating nor rejoicing. He talked about his deeply thoughts about the war.
He said that the "scourge of war", for him was divine punishment, because of the sin of slavery.
As he said: "With malice toward none, with charity for all, with firmness in the right as God gives us to see the right, let us strive on to finish the work we are in, to bind up the nation's wounds, to care for him who shall have borne the battle [...]"