Thats intentional. You are supposed to lure them to the other side to kill them when you run back
Viewing post in Zombie Throwout (Mashup Game Jam 2) jam comments
you can do this using clones. Also, I broke your game. when you take damage from a zombie it disappears forever, if you group them up by running circles around them, you can destroy them all and only take 1 heart, making there be no more zombies. (in my previous post I assumed you were already using clones)