In his early years, Masamune was a sick boy. Having lost his sight in his right eye to smallpox, he was seen as unfit to heir the throne, and many started to see his younger brother as a better fitted Daimyo. After realizing he was viewed as lesser, some day he plucked out the eye since it was useless to him anyway. It is believed that the adversity he endured as a child, may have made him the ruthless Daimyo that he was.
As said, Masamune was a ruthless Samurai. As soon as he was able, he began to take over the surrounding lands that his processors had made