Calling the markings on the casings, "antifa" is premature, in my opinion. The arrows are a Helldivers II reference. "Bella Ciao" appears in Far Cry 6 and is featured on a Groyper Spotify playlist. (The suspect cosplayed as a Groyper meme, and Groypers hate Charlie Kirk, so it adds up.) UwU is a reference to furries.

My guess is he's a terminally-online gamer/channer/Groyper and he was trying to troll news organizations into broadcasting his spicy memes. But this is just my guess, and until we get more evidence, my guess is worth about as much as your "antifa" theory, i.e., zilch.