Someone who is angrier than I am?

The other day, I was driving to my cooking class (somehow a weekly class has taken over my life), and I saw a bumper sticker that said:
AVADA KEDAVRA
(if only)

I had to think about this one. Who would have the killing curse on the back of their car? I mean, drivers in California can be pretty annoying, but I've never actually wanted to kill one. And, if I'm the one reading the bumper sticker, aren't I cursing the owner of the sticker?

And what was up with the parenthetical "if only" bit. Is that a wish that Harry Potter's world were real (I'm pretty sure Universal has that under control), or is that a wish to be a witch or wizard?

The real kicker of this whole thing was the other bumper sticker on the SUV: Official Handbell Transport Vehicle.