I played through the whole game, and you've done a nice job here! Here's a more in-depth review:
Design [4/5]: I'm very hesitant to call this a shooter game with only one bullet. It really just feels more like a stealth game to me than a shooter game. Having said that, it's nice that there are two phases to the gameplay. The first phase is complete stealth, the only goal being to get to the gun while avoiding enemies. The second phase is hunting down the enemy. My biggest complaint about the design is that the second phase is completely trivial compared to the first phase. Once you have the gun, it's pretty hard to get caught since you can just shoot the enemy from so far away that it can't even see you. I wish that the gameplay changed in some way once you had the gun. Maybe collecting the gun suddenly wakes up some sleeping enemies? Or maybe collecting the gun changes the layout of the level in some way? Right now, things are so trivial once you have the gun that the gun might as well have just been an exit door to the next level (although that wouldn't have fit the narrative you were going through - but that's basically how the gun functions mechanically at this point). That criticism aside, it's a nice game! The levels are nicely designed. All of them are challenging, but fair. You can always plan ahead and analyze the situation before you have to move into it. The art style clearly communicates what is an enemy and what is part of the level. Nice job!
Adherence to the Theme [4/5]: It is a shooter game with only one bullet, but I think it's more than that. It's really a stealth game. I don't think the stealth aspect fits the theme, though.
Originality [5/5]: Cool idea! I've seen lots of stealth games, but most of them take place in cramped buildings with human guards. This one really makes you feel like you're on the hunt for a creature far more threatening than you are.