Basically Stephanos told the leaders of the community that inwardly they were hardened by materialism, their hearts were not transformed, despite being official keepers of the Word of the prophets and God, and that they would not recognize the Word of salvation if it hit them in the face.
So they hit Stephanos in the face with stones, and the stones splintered off his face of an angel into so many pieces, thus spreading the message of the Word to Samaria, Ethiopia, Damascus, Antioch, and to the entire world.