Gakuen Mokushiroku

It shows how situations can distort a perfect "normal" human.
"Normal" i.e. the norm of how civilized humans are supposed to behave

Rather then distort, I would say that that it brings out the other side of humans.
A side that we have kept hidden in order to blend in to the world.

After the world is swarmed by zombies, what is the point of blending in to the destroyed world? The reason to keep the savage side of humans in check is gone. All that's left is to go wild in the broken world.

Almost every chapter is filled with gore. Killing to survive. In the normal world, people kill each other a little with words and indirect actions. A world without order breeds physical violence i guess. Killing is not legal and is not illegal in that world, so why keep yourself from killing?

Still, a tiny bit of conscience still makes those survivors guilty, but its gone in a split second when they kill again.

A good manga if you are looking for gore and more gore.

Higurashi Koro no ni

I like this series. Similarly, it shows how situations and people around you can distort your personality. But it is done in a more complicated manner. Its almost like a complex equation that you have to solve. You need patience and probably some wit to understand it. Or you can simply cheat and read explanations some other peeps came out with.

I think i like animes/mangas that plays with people's minds huh? lol