Cool concept, but i have few concerns:
1) I think, that bullets should leave trace. It would make them easier to... Trace actually. Especially when there is a lot of them on screen.
2) The game kinda lacks feel. Try to add some animations and/or effects.
3) Some music would be nice.
Also I've found bug on level 3. Sometimes the doors were open after my death.
Anyway, keep up the good work :)