It's a great idea, but it just needs a few more things in a second (which i hope you will make). Needs an incentive to kill people and some sort of treasure or items. 4/5 but with improvement it could definitely make 5/5.
the last feature is a bit dissapointing since you never run out of ammo if you can get gold on every level anyway. otherwise 5/5 looking forward to multiplayer.
the ending makes no sense and i have no idea why there are roadblocks in the sky. Also the difficulty is so easy until the spacey bit then it is so hard. 4/5 except for the ending and difficulty