Its toxicity is probably more lethal than the explosion, which would be a fireball without much concussion as it can't ignite until it mixes with air. If you really wanted a violent explosion from a compressed gas, compressed acetylene in a plain cylinder would do that, and all it would have to do is fall on the floor. Acetylene welding cylinders are filled with a carbon material to absorb small internal explosions caused by jarring, preventing the entire cylinder from detonating. Of course, filling and transporting the cylinder without killing yourself in the process would be difficult. As would filling a small cylinder of pure hydrogen sulfide gas. Where are you going to get it? The companies that sell it don't and won't sell it to individuals, they sell it to companies that use it for industrial purposes.
If I were a bad guy making an amateur bomb, I would personally opt for a pipe bomb, which can be made with some threaded steel tubing, end caps, and ground (powdered) fireworks propellant or model rocket engine propellant. An igniter would be an electric model rocket engine igniter, rigged to a battery and a contact closure device that closes when the door hits it. Dirt simple to make with easily obtainable materials, and relatively safe to the bomb maker.
Wow, okay, thanks for the tip, you're an angel for suggesting the acetylene. I'll probably end up using that instead.
As to your questions, this is why I've chosen to set it up the way it's been set up:
It's one of the protagonists. A little backstory: former swamp hermit that's lived life in almost complete isolation with her mother. They're from a place called Pecan Island, LA, and she's never left the town until recently. If you look up PI, you'll notice very quickly that it's the epitome of isolated—the small town of small towns. Taking this into account, I figured that she would be a practiced trap-maker, given that most of their food would have been caught using things like gator and stranglehold snares. It's plausible that she's read books on making bombs, but I would have to think of a way for her to have acquired that sort of literature given the location she's in at that point in time.
The story starts out months after she's absorbed by her extended family, who do have access to industrial material. She found the can in a cellar stash (it was her uncle's at one point, who probably swiped it from one the plants belonging to their associates, though she doesn't know that.) The witch that is helping her is under the illusion that they're pulling a prank. He believes that he is going to make the victim smell like rotten eggs by taking the hydrogen in the air and transforming it into hydrogen sulfide (still struggling with the magic system here. I was never good with this stuff, as you can tell.) The protagonist will set the explosive trap, and when the witch goes to make the sulfide, an explosion will occur, thereby allowing the protagonist to pin the blame on the witch by making him believe that he screwed up and ignited the compound he made.
Now that I'm writing it all out, I think that you've got a very valid point. Given that the can would be so difficult to get outside of industrial sale, and given that I would have to do a whole bunch of explaining to make it feasible, a pipe bomb would be a more pragmatic approach. If the victim is less than ten feet away from a pipe bomb, would it be more realistic to say that they died, or that they were badly injured? Would the house around the victim catch on fire?
So long as the outcome is death for the victim and the house going up in flames, I'm open to all suggestions. Though, if pipe bombs explode to the point of leaving little to no evidence, I would have to change the entire sub-plot for this section of the story. With the can, it was found in the remains of the house, and that leaves a trail back to the protagonist's family, which is what I need for the plot. A pipe bomb could lead back to the protagonist, given that the shrapnel would be left around (as I understand it.)
I do apologize for the long-winded reply. I'm sure that you don't care to hear all of the nuances of this story, but I just want to make this as realistic as possible, despite the fantasy elements in it. I suppose I could just simplify the chapter by taking the witch out and just having the protagonist plant a pipe bomb, thereby eliminating the need to even mention sulfide or acetylene. It will mean that I will have to scrap the first half of the story again, but I'm glad that the mistakes are being caught before I'm 50,000 words in.
