This is going to sound similar to the comment down below, but here goes
I like the idea and how simplistic it was, the in the end it turned out pretty hard for me to get into the rhythm and it proved a little frustrating. The lack of any sound also didn't help.
Perhaps some sort of timer at the top of the screen or some blip or audio beep would have helped alot with this.
Honestly, although I dont really like the state it is in now, I still see quite alot of potential for this game and if fixed up right, I reckon id like it quite a lot. Add some music and some indicator of when you're going to shoot next and really you're good to go.