Besides the point that it wouldn't be possible to use mustard gas for this purpose:

Regular pepper spray can be used to deter would-be attackers. (Let's downgrade this to "attackers," make it a little less open to a sympathy/emotional argument. Besides, why limit it to rapists, why not all potential attackers when you're hanging out in a less-than-safe place at night?) It can also be used as a weapon by "bad guys." Can you name a single thing that can be used to deter an attacker that can't also be used by an attacker? Maybe, like, a dog? But a would-be assailant could always train an attack dog as well.

Bitcoins - I don't use them. But I feel your entire argument discourages innovation and discovery. Why do I "need" to have something in order to justify its existence? I can't justify my own existence. I am just lucky enough to be on this earth. Do I need to prove in some way with my life that I "deserve" to exist? (Answer: No.) Bitcoins are an innovation, an experiment. A "let's see what happens if we poke this thing" sort of situation.

You are basically saying that if something exists that fulfills a purpose, why create a second thing that can also do that thing? My answer is: why not? You don't know how you can improve upon an existing product unless you try new things. That includes creating failures.

And, by the way - initially, you could get bitcoin for no cost or (really) for very little work, which was done by your computer. Can't do that with other currencies.

We should be able to experiment and we shouldn't have to justify our experimentation with "efficiency" and "purpose" and "reasons to justify its existence."

Consider Bitcoins a piece of economic art, if you must.