https://en.m.wikipedia.org/wiki/Attempted_assassination_of_Donald_Trump_in_Florida

Routh had a history of political activity online and offline, with his political engagement dating back to at least 1989. Voting records showed that he did not vote in the 2016 elections, but in a 2020 Twitter post, he claimed to have voted for Trump in 2016 and later regretted it. In a self-published ebook in 2023, he wrote about his supposed previous support for Trump, saying, "I am man enough to say that I misjudged and made a terrible mistake."

He was just upset that Trump did the things Trump said he was going to do.