Remember, Hack and Slash is only for when battle is being done. It's when you and an enemy are trading blows in pitched combat.
When someone is 'Surprised' (i.e. you've just jumped out of the cupboard and stabbed him with his own breadknife) or 'Defenseless (i.e. you've tied him up with his own rope, stuffed him in the cupboard, and are about to stab him with a bread knife), you're not in pitched combat, you're not Hacking and Slashing. What you ARE is the Fogbottom Strangler, and if you don't leave I will certainly call the police.
Generally, one of two things happen: if it's the sort of situation where you'd get one hit which may or may not be fatal (jumping out of the cupboard), you automatically deal damage. However, if you're in a situation where this one hit will kill your foe (i.e. you put a crossbow to a guy's head, he's not Jackie Chan, and then you pull the trigger)... well, he's dead.
This is true for any class... Backstab just gives you extra options.