Nice Idea
This looks like it has a lot of potential, so here are some ideas:
1. add different kinds of flood
2. make a points system, because without points, this game is useless
3. fast kills = combos
4. better death animations
5. cut the lag
6. add good music
7. make them go in different directions
8. add unlockable weapons that you can get by finding them behind the destructable wall
9. make the screen move, so you don't always stay in one place
10. draw everything better, especially the flood, make them look scarier and more detailed
There you have it, now improve!