While you're linking to youtube, you might checkout the homemade flamethrowers. I can't claim to have made one but plenty of my friends have (including my school physics club). The mechanics of a flame thrower is just a squirt gun + a match. I can buy propane "flame throwers" as is at the local hardware store (used for burning weeds).
Why are people making all these flame throwers? Because something that shoots jets of flame is freaking cool. As far as I could discern on a quick google search, none of them have been used to commit murder.
What I personally find horrific is the idea that anyone would be so afraid of their fellow citizens that their first assumption on hearing they have access to a projectile shooter/flame maker/etc. is "OH GOODNESS HOW ARE THEY GOING TO USE THAT MURDER ME?" I realize unhinged people are out there, and will do bad things, but there are also bears in the woods which could find their way to my house and easily maul me to death. But the statistics are low enough that I don't worry about. I suppose my luck could run out some day, but trusting my fellow citizens not to murder me has worked so far, and I wouldn't care to live any other way. I like the idea of a society and a government that assumes I have good intentions until proven otherwise and I consider it worth some risk to have it even if I am not personally a person who is interested in owning a weapon.