Killing NPC's in Dark Souls

My strategy for meeting anyone in Dark Souls is:

1. See if they have anything I can use, if so, get it.
2. Put on best armor and equip best weapons and spells.
3. Kill the NPC, if I can.

I have a feeling that this strategy will come back to bite me in the but later on, but killing them is so rewarding. Which NPC's should I not kill? Based on loot, items, power, and helpfulness in the game.

Solution 1:

The only NPC whose death has no impact on anything would be the Crestfallen Warrior - the guy sitting at the Firelink Shrine bonfire, suggesting you to ring the bells.

In general, if you are going to clean up the NPCs, do it right before NG+ so you don't have to play the game without their services while getting all their "only-when-killed" stuff such as the Blacksmith's Hammer and the Uchigatana. Otherwise, make sure they have no services you will consider using before killing them (i.e. buy all their weapons, spells, and stock up on their useable items).