Again, Context.
The LDS cult had been terrorizing the people of Illinois and Missouri for years with their Navou Legion and their Danite terror squads, including the attempted assassination of former Missouri Governor Lilburn Boggs.
en.wikipedia.org
They had watched Smith evade justice for years, from when he was convicted of fraud in 1825 in New York to the Kirkland Bank Scandal to the expulsion from Missouri, and just decided that the legal system wasn't going to deal with these guys, so they had to.