Really good game! Very solid design. The only thing is that there is a big incentive to just hide and kill everyone with the needle. Not sure how that would be fixed. Maybe with a scrolling screen, forcing you to keep moving? O some enemies that hit you from anywhere if you stay still?? Or maybe the enemies drop things you have to pick up that disappear with time, rewarding riskier plays.
Anyway, very good game. Good luck!