To Short
Make it have more than just two levels. The amount of people to shoot isn't that fun.
Also, make it so that there's a total ammo amount. That would make it more challenging. Secondly, include more than just one person per level. That get's boring.
My third thing, is that you could include types of weapons to shoot people with so that people are brought to justice w/o dying.
My last thing, is that you could have some missions where you can't kill the person but instead, injure him/her in a spot where him/her won't die.
Those are suggestions. Yes, keep making it and It'll be a winner for sure.