I think it's actually quite good. Reminds me of Alien Breed on the Amiga. The story element works extremely well and the battles are intense, since the infected, needs multiple shots to kill. The graphics work well, but the AI could use some path finding, as they get stuck a lot. However, I don't know if they gang up later, so maybe it's fine. Otherwise you could give them an intelligence level, and if they pass the intelligence check, they will avoid obstacles better? But I think you should ramp up the story element. Make it personal. How about if the main character talks/comments on what he sees. "Am I the only one left?" etc. maybe find/interact with survivors?
I definitely think you should keep working on it :)