Wasn't LDS leader Sydney Rigdon the first to use the term "extermination" with reference to "gentiles"?
Actually, the term was first used in the original draft of Doctrine and Covenants, then called the Book of Commandments, in reference to a prophesy calling for divine extermination of gentiles from the holy city of Independence, Missouri. Many people believe that Rigdon wrote that portion of the scripture.
The gentiles went completely over the top by burning the Mormon printing press after learning of their predicted fate. Smith and Rigdon went completely over the top by leading the disastrous Zion Camp expedition organized for the purpose of retaliating against the gentiles for their destruction of the press. Then, following their escape from Ohio (after engaging in a massive bank fraud), Smith and Rigdon embarked on a war with their own believers (see Rigdon's Salt Sermon and Bancroft's First Mormon Civil War), and the gentiles (see the Missouri Mormon Wars of 1838), before eventually fleeing the state (following their jail break).