No triggerred punishment, such as this, could ever be unfair. Don't want to face the punishment? Fine, don't do the action that triggers it. Simple.

In this case, though, I'd still be worried for the user because diminishing returns kick in rapidly against multiple attackers; you'll disable one relatively easily, but if there's more than one of them then it's probably going to be even worse for you (Especially if they're armed, I'd presume). Still, any protection is better than none.

But then, I'm on record as thinking it would be a better world if women had the legal right and ability to kill anyone who tried to rape them - if the law won't protect us, we have to protect ourselves.

Hell yes! Well, if everyone had that right.