It's just speculation. No real motive has been determined. https://www.latimes.com/world-nation/story/2025-09-12/what-we-know-about-the-suspected-killer-of-charlie-kirk
LA Times wrote this less than 15 minutes ago:
Investigators are still looking through evidence and trying to determine a motive in the slaying.
The only things in writing are the words etched into the bullets, and those are mixed. He wrote an anti-fascist statement on a bullet casing and other text on others, but nothing to indicate he had liberal or conservative ideologies. If anything, he appeared to be mostly mad at the whole system.