I think it was Lindsay Ellis who speculated that in the case of people like Lee Harvey Oswald (and potentially the guy who shot Lenon), it’s about notoriety. Oswald was a loser, who couldn’t hold down a job, thought of more of himself than deserved, was unhappy with how his life was going and wasn’t particularly bright. Due to his time in the military though, he could shoot a rifle.

Time and time again, there a plenty of people who are either contributing nothing to society or less often, are forgotten by it. They become bitter and in some cases are also mentally unwell. So they decide to make their mark by killing people. In the case of celebrities and political figures it’s a mean of forcing their way into the history books. I’m sure some of the time there’s an element of political motivation but it feels secondary it a lot of cases.