They were killed because of the testimony they persistently maintained to the Word. It is a legal term signifying their witness to the Word that God had revealed. They proclaimed Christ as the one and only King and Savior. And , they persisted in this testimony. The testimony of Christ cannot but provoke opposition if it is held forth faithfully, clearly, and persistently.
Blood has a voice and cries out for vengeance.
But surely these words also reflect the common cry of the people of God. Note the language used . Note the term by which God is addressed . A. White robes are given to them. They are told that they must wait a little while longer before their prayers are answered.