Agfdhyk

I read a story a while ago about humans, as a race, leaving Earth and searching for intelligent life. For a long time, they can't find it, then they stumble upon the remains of a dead sentient race. They find more dead races, then conclude they're the only intelligent life left. One day, they pierced a veil and encountered an alien ship, only to get into battle with it. Later, the humans find out they live in a region of space where every intelligent species goes insane and ends up killing themselves. Humans managed to survive because they are born insane.

This is a set of 4Chan stories, "Veil of Madness". To quote the TVTropes entry for "Humanity is Insane":

When humanity finally journeyed into space, they found several extinct alien civilizations, some pre-industrial, some which had expanded into space empires, but eventually they had all succumbed to some mysterious internal destruction. When humanity finally finds an alien species that's still alive, an aggressive reptilian one still in its prehistory, it quickly turns out that the entire race is psychotically violent to the point that spontaneous suicide is common, and they probably would've died out already if they didn't have such a high birth rate. It's not until humanity has its first disastrous encounter with an alien space ship that the truth is revealed: Earth is located in an area, encompassing around 3% of the galaxy, that is known as the "Veil Of Madness" because every species that evolves here (as well as non-native aliens coming into the area to colonize planets) are eventually driven insane by some kind of Negative Space Wedgie. Through some quirk of evolution, humans have just enough sanity that our species haven't succumbed to complete self destruction and are capable of rational behavior, but the fact that our species originated from the Veil means that the rest of the universe is terrified of us. Eventually, the humans give up on trying to convince the rest of the galaxy that we aren't roaming space murderers, instead using our scary reputation as an edge in negotiations and diplomacy, playing up the Terrifying Psycho Alien image by wearing scary power armor and distorting our voices. The result is that the other space-faring civilizations view humans as "scary and dangerous but not unreasonable".