I never tried killing friendly NPCs in BD. If clicking on one with weapons drawn doesn't give you the option of attacking, you could try the swirl attack nearby (not sure if it will hit friendly NPCs, though). An area effect spell that doesn't need a target may do damage, but I don't know of any available in act 1 ('elemental attack, spread/circle, missile/instant' spells are available starting in act 2). With 2 main characters and optional summoning dolls, though, area effect spells may have been designed to only effect hostile creatures, so mage's couldn't catch teammates in their attacks.
If you have any bottles of poison you can try tossing them at the NPC's feet.
BF merchant items should be regenerated each time you enter. They can hold on to stuff you sell them, though, which stays at the top of their inventory list. Other than that, the inventories should change somewhat (the type, number and quality of items should stay roughly similar for each merchant within each act).
