A nice idea, in theory, but the game in general is very badly made. The pure fact that the helicopter only comes when you kill 1000 zombies is just pure stupidity, and that the game restarts as soon as you win and you're forced to either mute your speakers and type comments with the knowledge that you're loosing constantly. Also the zombies and bullets aren't going for you, which makes this even more shit (pardon my French). The second a zombie spawns, or a bullet drops once you kill zombie, they just go to your last location and stop there. This makes it incredibly easy to farm bullets as you can kill multiple zombies with a single bullet if they are close together. It is very easy to win then if you just spam your mouse or enable an auto clicker and make your character do 360s all he time.
And the fact that it goes from 10 zombies to 1 million zombies in like a single minutes, just shows that you know people would never play it if the same amount of zombies kept coming until your reached 1000 kills.
It would have been a tiny bit better if you added a menu screen, and different modes like a survival mode or changing the score to a timer. This could've also been determined if you could choose a different difficulty, and maybe even added some different types of zombies.
Overall a bad game, but with subtle improvements can be a nice time killer. And maybe even a very nice addition to a full zombie survival pixel game.