They need to make him into a communist even if he wasn't. Too many reactionaries agree with this and it's making the bourgeoisie uncomfortable.

So he gets assigned that even if he wasn't. They use it to crack down on the left which is what they want and to make him out to be the right kind of public enemy though still a muddled one with a muddled definition of communist in the loose "Democrats are communists" kind of way that still gives enough cover to justify moving resources away from the right-wing extremists to surveilling the left instead and to pushing that narrative along with a bill through congress or executive order. If he was a raging swastika wearing Hitlerite and openly so online through weeks before the shooting they wouldn't be able to do this but would instead just bury it and call it a random act of violence without ideology but if it's at all muddled and there's even a chance of pushing this angle because of incoherence they were always going to and at the very least say he was poisoned by communist ideology into doing the deed.