I normally don't unless its a shop keeper that can buy my stuff.
My character /vampire lord is an anti - hero; if he can exploit or use something, or if an individual has purpose than he spares their life.
I think the only kill I regretted was accidentally getting Eorland Greymane killed when I was killing off the guards for their armor and weapons, and Eorland apparently ran to my aid and was killed by a guard.
So I had to re load my save file. But than again, there are other people I can do deals with including that dremora merchant you can summon from the Dragonborn DLC.
So, than again, I probably will have my character use his vampire lord form and kill all the guards and if Eorland wants to run into a sword than by all means that will be on him.