The world has changed and if you want to survive, rescue as many scientists as you can and kill all nasty creatures that are heading your direction. Upgrade your weapon after every fight.
There are many scenarios to the end of the world and one of them is mutation. Imagine if you see a mutated spider or any other creature, that behaves in a completely different way. What would you do in such situation? that's right, you will kill it! So, grab a weapon and spare no mutant on your way.