This is my theory. Ryan Routh, the alleged wannabe assassin, was a strong supporter of Ukraine, who actually traveled there and tried to recruit foreign troops to fight for Ukraine. He expressed a willingness or desire to die, fighting for Ukraine, more than once, in his social media posts, self-published book, and an interview with the Guardian.
So Routh figured he could go to the front lines and try to kill some Russians. OR, he could kill Trump. Trump's idea on the Ukrainian conflict, which I strongly agree with, is that the USA and Western Europe should bring the two sides to the negotiating table and come up with a cease fire or settlement that leaves unoccupied Ukraine secure and allows Russia to save face. Routh may misinterpret this as Trump being a Putin puppet. He probably thinks if Trump's elected, the USA will betray Ukraine and the Russians will win.
If you feel so strongly about supporting Ukraine that you want to die for the cause, why not take out the traitor Trump instead of going to the front lines and dying in battle? That's a lot more bang for the buck, if he viewed his life as "the buck." In Routh's deluded mind, that was the rational thing to do.
So I picked "lone, kooky gunman."