It is a commonly-held story that his mother had attempted to poison Masamune before he left to join the troops of Hideyoshi, and Masamune put his younger brother to the sword instead of executing his mother for punishment.
参陣前に母親に毒殺されそうになり、母親を成敗する代わりに弟の伊達政道を斬殺したという説が通説となっている。