Not joking, this has happened a couple of times. The guy in the UK that did this just before the Brexit poll had zero history of being either political or right wing. Suspicious as fuck.

huh?

The perpetrator of the attack was Thomas Alexander Mair, a 53-year-old unemployed gardener born in Scotland.[19] Mair had mental health problems,[20][21] though he was declared sane in the moment of the crime.[1] He believed individuals of liberal and left-wing political viewpoints, and the mainstream media, were the cause of the world's problems.[21] Guardian writers suggested that he targeted Cox, a "passionate defender" of the European Union and immigration, because he saw her as "one of 'the collaborators' [and] a traitor" to white people.[21]

Mair had links to British and American far-right political groups including the neo-fascist National Front (NF), the United States-based neo-Nazi organization National Vanguard (the successor to the defunct National Alliance) and the English Defence League (EDL); he had attended far-right gatherings and purchased publications from the National Vanguard and other outlets,[21][22][23][24] to some of which he had sent letters and expressed support for South African apartheid.[25][26][21] In his home were found Nazi regalia, far-right books,[19][27] and information on the construction of bombs.[21][24] He had searched the internet for information about the British National Party (BNP), South African apartheid, the Ku Klux Klan, prominent Jewish people, matricide,[19][27] white supremacism/nationalism, Nazism/Nazi Party, SS/Waffen SS, Israel, mass shootings, serial killers, Frazier Glenn Miller Jr., William Hague, Ian Gow (another assassinated MP),[21] and Norwegian far-right terrorist Anders Behring Breivik (about whose case he collected newspaper clippings). He also owned Nazi iconography and books and films related to the Nazis.[28] A police official described Mair as a "loner in the truest sense of the word ... who never held down a job, never had a girlfriend [and] never [had] any friends".[21] The Guardian said that he "appears to have fantasised about killing a 'collaborator' for more than 17 years, drawing inspiration from" David Copeland.[21]

Seems he was just a nationalist who also had violent tendencies. Wikipedia also mentions he had a lot of mental health problems and the night before the attack he tried to get help for his depression, assuming he was on antidepressants it's understandable what he did to be honest. I've known a few people who've been on SSRIs and all of them have told me they had violent thoughts whilst on them but never while off them, specifically fantasising about killing politicians and seeing if they could get away with murder.