The world tendency is a very annoying thing in Demon's Souls. Basically, if you die while ALIVE or if you kill NPCs, your world tendency drops to black. Which spawns black phantoms (really, really hard to kill) and black phantom versions of mini-boss like NPCs. In Black World tendency you get better drops but your HP is lowered and the mobs are harder to kill.
Your world tendency goes up by 1 each time you kill a boss. It makes the enemies weaker and gives you better HP, as well as access to the normal version of some minibosses. Each version drops a certain item and can only be killed once. Read the wiki for more info on this.
My advice...spend the game as a ghost. Kill yourself in the Nexus as it won't impact your world tendency that way, at least on the first playthrough. After that, on subsequent NG+s you can affect your world tendency to farm the items you need.
For more info, I really suggest the wiki. There's a lot of stuff there that will make your life a LOT easier at the start and you'll avoid frustrations.
