Okay game
Very simplistic storyline that doesn't really explain much. Here are the problems.
1) You provide 4 different weapons. The pistol takes several shots. The shotgun takes a few shots. The machine gun takes one burst (one click.) The sniper gun is too hard to target anyone with. You never run out of ammo for the special guns, so why would you use anything but number 3, the machine gun? There's absolutely NO reason to use any of the other guns. Make it so you either get a new gun each stage, or that there's limited ammo for them.
2) The bad guys would spawn in highly unrealistic places. On top of a gas station, from behind the side of the top of a building, and hell... even rising from the ground like demons from hell. For the ones not coming from behind cover, have them run from the side of the screen or something.
3) Very, very easy. Each guy takes one hit to kill with the machine gun and your guy can take tons and tons of bullets.
4) There's no ending. You finish all the stages and that's it. It's a huge letdown.
The graphics are there, as are the core game mechanics. Fix the above items and the game would be drastically improved.