The metal slug universe.
I mean, the world is in a constant war where the rebel side is willing to do anything, they have destroyed villages and small towns according to guidebooks and art books, they have placed explosives in toys to give them away, not to mention taking hostages and placing them in horrible places like bonfires or hanging in the middle of the jungle.
There is an island full of the most hostile fauna, crabs, mosquitoes, worms, eels, jellyfish all of them colossal.
The world has been invaded by aliens TWICE.
There is an Egyptian curse that has been unleashed (it started in the second game and you can see how far it has progressed in the fourth)
The worst of all is the zombie virus. It started with an asteroid that shot down a plane and killed and turned most of the crew into zombies, leaving the rest disabled. The rebel army brought its scientists and soldiers to protect and investigate the meteorite, but they ended up converted due to exposure. In the fourth game we can see how this virus reached a city turning all the inhabitants into zombies, to discover that it was the rebel army who brought this virus possibly to experiment on them.