If the words of the speech are properly recorded (this is not clear since the speech was only written down years later by a biographer who tried to reconstruct it from the memories of those who had been there) Henry was speaking out because he felt that it was his duty to do so.
Henry does not say anything in the first two paragraphs about what the specific issue is other than that it is an issue of freedom versus slavery. Everyone at the convention knew what they were talking about so it was unnecessary to spell it out.
What Henry does say is that he feels he must speak, regardless of whether it is deemed to be proper. He says that if he kept silent he would be
guilty of treason towards my country, and of an act of disloyalty toward the Majesty of Heaven, which I revere above all earthly kings.
Henry, then, feels compelled to speak because he feels it is his duty to tell the truth as he sees it.