I killed the merchant in undead burg and got a sweet sword so what other ones have great loot or are just mother fuckers and will screw you over

I would say don't kill any of them unless you know you wont use any of their services. I would also say never kill a blacksmith.

The Blacksmith in Anor Londo drops a pretty sweet hammer.

If you kill Fire Keepers then you can use their soul to upgrade Estus Flasks at another Fire Keeper. But killing them will permanently extinguish the bonfire next to them.

There's a certain one that I always kill as soon as I can, but I don't want to spoil anything. There are a handful of other NPCs that you might or might not want to kill, depending on your build.

Just let them be until you get ready to fight the last boss, then kill everybody.

All the blacksmiths drop hammers except Rickert, I believe.

@ajamafalous: Well look at you! I thought you were wiser... but I thought wrong.

Yep!

All of them.

Start with Andre.

You mean you don't kill all of them?