I now have a level 30+ halfling thief type character and I've been doing pretty well with a couple of different weapons for backstabbing. Against "soft" targets, those without armor, I use the "Well Balanced Sword" Magnus made me. Against bears, monkeys, spiders, etc. it only takes a few hits to kill stuff. When I go up against sentient type critters like barbarians, bandits, etc. the sword doesn't seem to do as much damage. I swap to my "Dagger of Speed" and I can mostly kill these guys in one round, though if I use the sword, it seems to take two rounds. BTW, against the weapon damaging golems, my thief is an expert bowman, using his high agility and quite a few arrows to finish them off. I'd destroy my "Dagger of Speed" on those damn golems in one round if I hadn't concentrated upon Archery as well. Can I expect to use these two weapons until the end game or is there any better backstabbing weapon to anticipate?
Ok just a few bits. If the npc is unaware more damage is done. Second if you reach master level in backstabbing then you can use swords and axes to backstab, but not before this level. and at Master level you are lethal at backstabbing.