Basically, auto_swap_neutral is an awesome feature, but it makes it so you can't successfully backstab a neutral or friendly creature.

Instead, I suggest that :s should work as a toggle both ways, so if you use it and auto swap is already on, it'll give you the "do you really want to backstab *foo*?" prompt.

Either that, or make backstabbing an applicable skill. Right now if you try to apply it, it just says "pick a suitable target."