This is a solid game idea, and impressive to do it in 100 lines of code! It probably just needs a few tweaks to get the difficulty just right. As others have mentioned, the boomerang needs a little work for pickup, but the mechanic of just letting it sit there spinning and taking out a line of zombies is just fun XD The music and art are spot on, but a few sound effects would also add a little extra to the game. Excellent entry! Well done!