To feel guilty, was a rarity. Given his past experiences and the lack of sympathy he had received from others as a child, he did not grow to be a sympathetic person. That was until the Reborn Oracion Seis had fallen, and he was forced behind bars for the third time in his life.
He thought about how his actions affected his comrades. His efforts to realize his father’s dream turned him into someone who could eventually grow to be as terrible as Brain was, through his eyes. He felt that he used his comrades. He had thrust his father’s beliefs onto everyone and influenced them to make terrible sacrifices for power. Macbeth felt regret especially toward his impact on Sorano, who had willingly given up years from her life in order to support their goal.
In the end, Macbeth realized he had to choose between his father, a manipulative and uncaring man, or the comrades that stood by his side throughout everything. Macbeth was able to accept Brain’s death. Although his feelings in that moment are extremely simplified by this: Words couldn’t express that gratitude he felt toward Erik, for doing what he couldn’t. He felt that Erik freed not only himself, but all of them from just one more heavy burden from their past.
This all plays a large role in Macbeth’s desire to ‘change’ himself for the better and not risk losing his freedom, after being released from prison one final time by Doranbolt.
Note: Obviously the OS still had their own individual reasons for wanting to do what they did and make their sacrifices. However, Macbeth still felt at wrong as a leader. Which is why the first thing he did once they were released from prison again, was say that each of them should be free to live their lives as they want and make their own choices. (ie: respecting Richard’s wish to live for love and not fight)