Why are we accursed?

I am confused about who gets resurected and who doesn´t.
At first I wondered why the NPCs and bosses could die for good, while the player and the normal monsters get respawned. This has to do with the dark sign,I think, which makes you immortal and is the reason why all the soilders went mad. I guess some people stay sane because they have a strong will or a firm grip on humanity.
So is the dark sign the cause of the hollowness or are there especialy accursed Hollows or do you get the darksign if you turn hollow? The last can´t be true, since that Oscar-knight and Soliare and others turn hollow, die and stay dead.
I would say, that only the player and the normal monsters carry a dark sign and the player is the only sane one, cause he or she is the chosen undead. But then who accursed us and what for?
Also, is every human in Lodran undead? Do you live, then die once, turn undead, die a second time, turn hollow?

The Undead is a curse and the Darksign is the brand of the Undead. The NPC's should be reborn if you kill them, as everyone in Lordran is Undead I think. But for game mechanics they are not reborn. If they turn Hollow, they will never regain their humanity, but the Darksign continues to trigger and they are revived, that's why Lordran has so many living Hollows. When the player is in Hollow form, it is close to being Hollow, unless you lose your OWN sanity playing this game and start doing insane things lol.

Probably for game purposes NPC's do revuve using the darksign but you killing them turns them hollow (if they weren't already). After the first killling they ruin theirclothes and start to look ike ordinary hollows.

The NPCs probably just poof somewhere else when they die hollowed. Somewhere outside of Lordran.

Nah they ragequit and dont respawn at bonfires.

Well, one time I killed that guy who sells magic and then he was back in that house he was first in later on. I think I absolved my sins between going back down there as well. It was weird