There are good ideas, and there are bad ideas. Filling an air horn with butane to turn it into a flame thrower? That's an extreme example of one of those things, but I can't decide which.
The best part of this video is the fact that the air horn still makes the standard sound when it's spewing out fire. Although I do wish it shot the fire a bit farther. I guess that's one for the next version, if this guy lives long enough to make a second version.