Zombie Shooter 2
Well where to start, maybe with WARNING that on modern PC this game can be hazardous to your health and I'm not joking.
ZS2 have weird screen tearing and slowdowns that can cause nausea and there is really now way to fix it. (I tried all possible fixes but still nothing )
And about game itself well it's still old Zombie Shooter where you kill hundreds of zombies with tons of weapons.
When you start new game you can pick from few diffrent characters with various background and stats (That don't really matter as you progress)
Weapon and Armor arsenal had been extended lots of new weapons and armors including power armors that basicly turns you into T-800.
Missions are a bit more diversified in one mission you are traveling in car as turret operator and in other you get to operate a Tank so there is little improvement comparing to Zombie Shooter 1
Overall game is worth playing if you have 2-3h of free time, also you can buy it for less than 1$ in many various bundles.
But bare in mind the problem I addressed at the beginning.