In the full story of a local, which is referenced on wikipedia—the writer who knew this guy well, referred to a 1926 election for the council or county office, which he ran for and lost— he felt the whole community rejected him.
In the writer’s opinion, this plus the “debt” he owed (having spent on stuff he didn’t need), his inablity to simply farm a single crop, plus his wife’s tuberculosis hospitalization, all these led to this planned revenge. As you say, he wanted to kill everybody. The building having been destroyed (if it had) he intended to drive up to the crowd outside and kill them all and himself ( his car, according to the writer who saw what was in back, and testified to it at the coroner’s inquest— was filled with all kinds of scrap, a box of over a thousand rifle shells and on top of that, dynamite).
So, in this sense he was a real narcissist politician, and cannot handle rejection. Writer also said that it was “his way or no way at all” and he would adjourn meetings. These behaviours similar to our most self involved politicians, who have to insert themselves in everything. Comrade Stalin.