No one knows 100% why Lee Harvey Aswald followed trough with this but it is assumed that it was because he didnt like the change and was a hard beliver in communisum
https://www.jfklibrary.org/learn/about-jfk/jfk-in-history/november-22-1963-death-of-the-president
https://www.archives.gov/research/jfk/warren-commission-report/chapter-7.html