Really cool idea, and nice challenge. I don't think I'll be able to beat this one today, but it's a skill issue and not a design issue. Really fun concept and well thought out.
If you continue to tinker with it, I must have missed where the "health" indicator was - I couldn't tell how many more bad guys could slip through before I'd get a game over. It was also sometimes hard for me to tell who was good and who was bad... maybe a contrasting color would help me to spot them better instead of just that gear symbol over their head.
Neat little game though!