http://urbanlegends.about.com/od/crime/a/wasp_spray.htm"
U.S. residents tempted to avail themselves of this Internet-recommended self-defense option by stockpiling wasp spray would do well consider that federal law prohibits the use of any pesticide "in a manner inconsistent with its labeling" (such as self defense). Likewise, some states forbid carrying substances for self-protection that aren't specifically authorized for that purpose."
Either get a weapon intended for self defense or do not carry one at all. And please read up on the laws before carrying anything in self defense. Proper use of a gun in self defense is within the law, use of bug spray in self defense will get you arrested, thrown in jail, and then likely sued by the guy who attacked you.
PLEASE DO NOT DO THIS!
The actual amount of times a gun is taken away from it's legal owner and used again him is so low it is really not worth considering... and that argument falls short when you are
9 times more likely to be killed by a blunt object (such as you can of bug spray in your hands, or the bad guys fists) than a gun anyway. Personally I would not want to allow him to get within 20 feet of my body.
It is also widely accepted in the community of concealed-carry-permit-holders and Police Officers that
a knife wielding bad guy within 21 feet of you has a very good chance to inflict fatal damage to you before you can draw and fire your gun. 21 feet! And that is with a gun on the hip of someone trained in it's use... and that wasp spray only goes 20 feet you say?
However, as I've said every time I post on this subject, if you are not comfortable with a firearm or not willing to take the time to train and practice regularly,
DO NOT GET ONE. If pepper spray makes you feel safe, then that should be your weapon of choice. If a whistle does it for you, then get a whistle. Only get a gun if you are 100% committed to spending thousands more on training classes and then spending even more each month (ammo and range dues) to practice the skills.