Fun time killer but ..
Poor control really took a lot of fun out of this game, nothing more need to be said on this issue since you already noticed and attempt to fix it. (Meanwhile, my fellow ngers: when you are struck/moving-on-his-own, let go of the movement key, then try hitting the each direction key once seperately. If you mention to hit the "right" direction key you are struck on you will be unstruck)
Gameplay-wise, it's too easy and too repetitive after awhile. The zombie AI have some bugs too, as zombie have to move forward for a short distance before they could turn (so you can just stand right beside them as they come out, wait for them to overshoot/pass you and shoot their back): Taking advantage of this flaw and upgrade every point on bullet means you are pretty much invincible while earning max multiplier bonus (18 is the max for me).
Pretty good 5min game but if you want to improve on this I suggest:
Smarter AI
More variety - different zombie with different "peaks", bosses every "x" level, etc
Mission goal - e.g: save "x" amount of hostage, take down "x"# of (purple/gold) zombie, etc
Better upgrade