Writing down feedback as I play:
The game opening with a loud baseball bat noise before the player can adjust volume is good, good games always deafen players and send them scrambling for the option menu. It asserts dominance.
I expected the tone to be more lighthearted considering that the game is about a kid killing green dudes with a slingshot.
I'd like to be able to hold the fire button to fire the slingshot continuously considering that the slingshot has infinite ammo.
Tried a controller just to see if it played better but the reticle is much easier to aim with a mouse, usually in twinstick shooters you rotate the character in a general direction instead of moving a reticle.
Let me quit the arcade minigame without having to die 3 times please.
The character moving off-center feels bad, I think it would work better if the camera became offset from the center instead so you didn't move closer to the edge of the screen when you move.
Sometimes the hurt noise is pitched up to presumably sound like a kid but sometimes it isn't and it sounds like an adult, seems unintended.
The pistol sounds like it's being fired underwater in another room.
The bloody footprint effect is nice, I only just noticed it.
The suburb level being a square you can roam around is dispersive, maybe when there's 10 or so enemies remaining you could have an arrow pointing at the closest one.
1 and 2 being previous and next weapon is unorthodox for keyboard controls, I'd rather have each number key select a gun, guess I'll stick to the mouse wheel to select different ones.
City part, I was searching for the last 2 enemies for 5 minutes so I decided to check if you can fall in those holes.
You can fall in those holes.
I used a grenade and picked up a molotov and now I can't pickup grenades. I think there's a limit on grenades in general instead of by types. Some UI indication would be good also regular grenades have are very hard to time right because of their long fuse.
The blood effect looks bad because it has a cooldown. I think you have a single particle in the enemy scene that is set to emit when hit which means there can only be one active at a time, a better solution would be to pool more than one or instantiate them as needed. It sounds like a small thing but it does a lot for weapon feedback.
School level, I like it more than wandering around looking for kills.
The fence here does not extend all the way down and I went out of bounds: 
Even going out of bounds I couldn't find the last enemy.
Boss got stunlocked by the m16 for most of the fight then I shot it a bunch with the shotgun and it died, not super interesting.
Was the last enemy the boss? I've been conditioned to clear levels and ignore area transitions so I thought the counter was broken.
I think enemies need to be a bit more interesting, I never needed to dodge and I barely needed to sprint.
The dog enemy could have a lunge attack for example that require you to dodge out of the way instead of being able to backpedal away from it.
I'm not a fan of the two city levels being so open and dispersive, I'd rather the levels be a bit more linear and have properly planned encounters, the levels being less open would also make it harder to backpedal away from everything.
There is a lot of ammo and it didn't feel like guns did anything much different from each other. There's no real reason not to use the shotgun or assault rifle for everything. I think a reload mechanic could add some ebb and flow to combat, right now you just kinda walk backwards while firing at everything.
The style is charming but I still have whiplash from the kid's parents getting butchered and then seeing those silly little goblinoids taking a car for a joyride.