You probably already saw that this is from the video game Helldivers. In Helldivers, you play basically as Starship Troopers infantry and the game world's propaganda says you're fighting for managed democracy against fascist bugs and robots, the joke being that you're obviously a fascist foot soldiers and you die all the time. You can call in airstrikes and orbital bombardments and a bunch of other stuff, and "catch this, fascist!" is something you might say before calling in an airstrike if you're kind of LARPing like you're taking it seriously. There's plenty of video game YouTube channels that make silly little videos goofing on the gameplay and that might even be a line from a specific one. Something like this:
ETA: dahobbs beat me to it. It's possible that he was a groyper and personally took the "irony" of the game to align with how "ironic" he saw it that a further right fascist would kill Kirk, but more likely it's just a joke the mass shooter fandom community already memefied and by using it in the shooting he's basically referencing a reference of a reference of a reference of... etc. They do this frequently now in attacks (e.g. the shooter who shot up that Catholic school recently wrote the Loss meme on the gun). Sometimes there's some sort of alignment with alt-right shit, but usually it's just nihilistic "jokes."