The correct answer to this open question is the following.
In my opinion, what compelled Wright, who knew the dangers of speaking out, to step up and testify against the murderers was his courage to stand for the truth and demands justice for the black people. Wright's actions and testimony made him a symbol of the emerging civil rights movement because he dared to stand up and identified his nephew's murderers, in a time during the history of the United States, where was so difficult to stand up and be counted if you were an African American. Indeed, what was expected for a black individual was to say nothing, to be quiet and accept the violations of their civil rights. So one could say that the merit of Mose Wright was to speak his voice loud in a time when African Americans were supposed to remain in silence.