Shoot the zombies and don't let them touch you! Every 10 kills gets you an upgrade to fire rate, walk speed, or you can heal 10 health.
This is a part of a weekly competition. You can find its competitor, as well as other weeks, here: https://scratch.mit.edu/studios/36389441 You can also find the original and possibly updated game here: https://scratch.mit.edu/projects/1136272478/