Summary: There is no doubt that the tragedy of Kerbala, when ascribed to the killers, is a criminal and terrible act. However when ascribed to Hossein (A.S) himself, it represents a conscious confrontation and a courageous resistance for a sacred cause.
Why is Hossein (A.S) regarded as the "the leader of the martyrs”?
It is because he was not just the victim of an ambitious ruler. There is no doubt that the tragedy of Kerbala, when ascribed to the killers, is a criminal and terrible act. However when ascribed to Hossein (A.S) himself, it represents a conscious confrontation and a courageous resistance for a sacred cause. The whole nation had failed to stand up to Yazid. They had succumbed to his will, and deviation and regression towards the pre-Islamic ways were increasing.
Passiveness by Hossein (A.S) in this situation would have meant the end of Islam as we know it. Thus Hossein (A.S) took upon himself the responsibility of the whole nation. The greatest tragedy was that one, who stood up for the noblest of causes, the defense of Islam, was cut down in so cruel a manner. It is for this reason that the sacrifice of Hossein (A.S) is commemorated annually throughout the Muslim world.
Source:http://alhassanain.org/english/?com=content&id=433