The title is a good joke. "Start Killing Slop" playing off "Stop Killing Games" flips the campaign's name and turns it into a comment on disposable games at the same time. Anyone who knows the jam smiles at that. And saying openly that it's your first jam and you made everything yourself is the right way to show up.
The red brick floor has decent texture and contrasts readably with the green enemies. The HUD is lean and in the right place, HighScore and Score top left, ammo at the bottom, nothing in the way. Controls are on the page and are exactly what you'd expect.
The main thing to fix, and it's a big one: the player is a FLOATING GUN WITH NO BODY. It took me a while to work out where I even was on screen. I assumed all the green shapes were enemies and that I was somehow off camera, and I only realised when I fired and the gun rotated. Drawing a body under the gun, even just a circle, fixes that completely and would change the whole first minute for every player.
Second: the enemies are irregular green blobs with no face and no facing direction, so you can't tell where they're heading or whether they're about to hit you. Even a single eye or a pointed front would help.
Third: I fired and the score stayed at 0. I don't know whether I missed or whether enemies take several hits, because there's no hit feedback at all, no flash, no sound, no number. Adding any one of those three would make shooting feel real. Also the first click on the canvas only takes focus rather than firing.
For a first jam the fundamentals are there, you move, you shoot, there's ammo and a score. The priority is simple: show the player, and confirm the hit. One note on the end of life criterion, since it's scored here: "the game will stay on itch" leaves it entirely dependent on the platform. A downloadable build would cost you nothing and fits the jam.