First of all good job on having submitted a game. I had fun shooting the zombies, though I did get stuck at the parking lot. I did not know if I had to shoot all the zombies, find the car or find some door. I did run out of ammo, I was two bullets short so I was not able to kill the last zombie in this level. Second, I liked the simple graphics, you two did a good job on that front. The music fits quite well with the general mood too :) Lastly, I suggest that you maybe use the first level as a tutorial or guide of some sort, so the players know what to do or expect.
edit: So I went back and replayed the game. I was suspecting I was missing something, and ofc I did not read your instructions for the controls XD (this is on me). In any case, I found the door and from there kept going.
A few remarks: - The game gets a bit repetitive after some time, though I understand that in a game jam there is only so much you can do. - I noticed that the zombies do not see you unless you enter a certain radius, this makes sense, but this made it too easy to kill some of them as I could snipe them from far. Maybe impose a greater number of bullets to kill if you are outside that radius. It's good that you made the zombies quit the chase when the player gets too far. - It was also a bit strange that a kill needs 4 hits with the briefcase or 4 bullets. Maybe melee attacks should be weaker ? These are small changes but they add some flavor to the mix. - Same remark about the radius for the boss. I shot him with ten bullets, I did not need to move.
All in all I really enjoyed playing. And I hope you keep working on the game and perhaps develop the story further. Though the zombie apocalypse theme has been present since forvever I found the attitude you imbued into the main character to be funny. What he says rises a lot of questions that I would like to see answered. And this is a sign of a game with good potential.