Fun game. User created levels would be an easy and interesting touch. Run a beta challenge under the pretense that the best user created levels will be added if you don't want to actually take the time to generate new levels.
On the right track, though this is still a beta.
A) Shop is useless as of right now. Its actually possible to win without upgrading (though a bit difficult). Either increase difficulty to make the shop necessary or do away with it.
B) I agree with MM, I don't know if i should be letting those innocent people die? Somehow incorporate this into gameplay for level score bonus to reflect surviving innocents?
C) You could probably do away with switching between weapons, I mean why would someone want to switch back to an inferior weapon?
Anyways good job for now but it still needs to be fleshed out a bit and polished.