So, I found this game! It's called "Alien Swarm: Reactive".
I have been finding it a lot of fun lately. It is very similar to zombie massacre in a lot of ways, a top down shooter, where you're shooting zombies.
Oh, did I mention that it is free to play?
Here ya go, http://store.steampowered.com/app/563560/Alien_Swarm_Reactive_Drop/