I don't think that normally I would like a game like this. I actually really did end up liking it. While it does come off as repetitive to a certain sense, it's actually quite well done. What mostly works is how you are so authentic to the style of the Metal Slug games. I've never played them, but I have seen a lot of tributes based on them. It's a wonderful style.
I love how you try to cause damage to zombies behind the one you're firing on. The all around graphics and animation are just really nice. The sounds work great too. I am starting to recognize these sprites. Why wasn't I playing this growing up?